Apologies for the inconvenience caused to you due to this issue.
Unfortunately we are not able to reproduce this issue. Could you kindly provide the following information to help us nail this down.
Kindly share the current version of Ai that you are using and the last update which reset the preferences. Also, please rename your Preferences folder and share this folder with us in a Zip file at sharewithai@adobe.com
Please do mention this User Voice link so that we can identify you.

Year after year for what seems like 10 years now. Everytime I have updated Illustrator... Beta or regular my workspace settings are gone. I set it everytime and make sure it in the list but then a new version comes around and poof gone! How much time has been spent just returning your workspace to a functioning one. Yea, adobe... give me your useless settings and setup but I WANT MINE. They make me move faster and I don't have to think about it. After an update I spend 20-30 min rearranging and finding the tools i use the most in my space they way I like to work. So for every update in the last year that is like several hours of lost production. Tell me who do I charge for this lost time... Do i bill you for making me redo my workspace? This is a serious amount of time that I can't get back, just wasted on rearranging stuff I already have arranged and then saved as a preset that goes away everytime I update. Why can't you just fix it. WHY is this a crowd voted thing. If you have the system in place just make it remember. It just laziness and I should get a discount or money back for the loss in productivity. Just make it work that is all we ask.

Same should apply to downgrading Illustrator, if possible:
http://illustrator.uservoice.com/forums/601447/suggestions/47264024

There is a similar bug that affect Beta versions of Illustrator, when updating leads to resetting preferences:
http://illustrator.uservoice.com/forums/939477/suggestions/47082262

Every time Ai Beta updates itself, and sometimes without, the Saved Workspace panels I cultivate are reset and have all workspaces cleared. Extremely frustrating to have to spend 30mins re-optimising my workspace every 3-5 days, as all of the standard template workspaces don't facilitate my needs.

Voting, but I am not sure about this...
Standard workspaces are standard... if you want to have your own — you better save a modified version of it under a unique name.
...on the other hand, Illustrator allows to resave them, and then tosses these changes away.

My past custom workspaces are gone, toolbars and controls default to off, and all of the windows that I had mounted on the right are gone. I reset them all up and try to save a custom workspace but that option is greyed out. It's almost as if the illustrator is treating my desktop as a tablet or something. would not have updated if I new all my workspaces would go away.

Same problem for me. Can't even create new workspaces. Very frustrating! Edit: Just did a completely fresh install and still not working.

AI will not let me reset or go into any of the display presets. Always reverts to Essentials classic with no reset option and no toolbar or panels. I have restarted my computer and it still does the same thing.

All my toolbars are missing. Updated to last version after that happened - no change.

RESET button on toolbars does not work.

only ESSENTIALS CLASSIC toolbar selection allowed.

I can't get mine to work either. This is so frustrating. And no help anywhere.

I think this is the same thing that is happening to my version of Illustrator CC 2020 as well. I usually need to resize the window to get anything to show up. Then when I switch between tool panels I have to resize again. A little annoying if you ask me. Hopefully someone can find a fix for this.

All my toolbars are missing. Updated to last version after that happened - no change.

RESET button on toolbars does not work.

only ESSENTIALS CLASSIC toolbar selection allowed.

Dear Adobe. The last month you made update for illustrator twice. And twice after updating ALL, I repeat ALL my setting just disappeared. Neither at the program it self, neither at the computer folders., nowhere. Just gone. All key shortcuts, all swatches, even the essentials! TWICE.
Feels like i using a pirated version...

Dear Adobe. The last month you made update for illustrator twice. And twice after updating ALL, I repeat ALL my setting just disappeared. Neither at the program it self, neither at the computer folders., nowhere. Just gone. All key shortcuts, all swatches, even the essentials! TWICE.
Feels like i using a pirated version...

what planet did the person come from who decided that .33 of an inch was a good grid size to use as a grid? why WHY!
UNBELIEVABLE

I am pretty angry about this as well. I work in screen printing and have a lot of swatches set up for halftoning, and when the units and grids get reset it ruins everything. This is completely infuriating. My carefully created and tested halftones go from looking like they are supposed to on my monitor to looking like plaid patterns. useless.
does bill gates work at adobe now?

me too.

Please stop reconfiguring our tools and workspaces every update. Work time is lost from opening windows I use everyday. Even when i spend time making my own workspace automations, it's overridden with every update. We are working professionals, who lose work time configuring our tools.

I have commented on this previously, but it seems to have gotten worse.
PLEASE, please, please stop resetting my preferences (window layout, tools, persistent color palettes disappearing, etc.) with each update to CC. It is very frustrating to have to spend time to do this so frequently.
It updates to 'default' each time...turns off rulers with every new document, changes to picas instead of inches, places windows in different locations.
VERY FRUSTRATING to have to do this so often.
Illustrator and InDesign have both reset my preferences with this most recent update (and the one before that, and before that, etc.).
I have saved a workspace, but am still having these issues.
I appreciate the continual updates, but this is just crazy that the software resets these each time.