Trick works with me:

-from good old "Control Panel"
-open "Tablet PC Settings"
-Click "go to orientation"

1. primary portrait
2. secondary portrait
3. primary landscape
4. secondary landscape

I found a workaround for this. I'm using Surface Pro 6 and had the same issue. So I followed this:

1 - Open the Registry Editor by typing Regedit into the search bar.

2 - Copy and paste the following into the address bar and click enter: ‘HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\AutoRotation ‘

3 - Now, check that the following values are there and properly assigned:

Enable: 0
LastOrientation: 0 -> (in this case I have 3 I guess that is Vertical (Rotated))
SensorPresent: 0
SlateEnable: 0

4 - Shut down and turn On again. I think this will disable the rotation sensor and avoid Illustrator to force landscape.

Regard! :)

Srry for my english lol

Seems devs dont care about its users nor do they listen to feedback. THey should start look at useful feedback instead of adding new features which make illustrator wack!

I find illustrator quite broken last couple years. I can hardly snap something properly without the need to zoom in to X100 procent before i get snapping to work properly.

Then there is SUPER annoying issue with the selection marque handles. You want to scale or more it and it starts to rotate. This is seriously the most annoying issue for years. Its said it was fixed many times and has been returning from 2016, 2017, 2018, 2019 and i think i already saw 2020 having the same issue

still wonder why this thing only has 150 votes?!?!
https://illustrator.uservoice.com/forums/601447-illustrator-bugs/suggestions/31995940-rotation-handle-tolerance-overrides-scale-and-sele

Don't think they'll be bothered to fix this

PLEASE FIX ADOBE!!!!

It's wild that this is still an issue. =(

Since @sjc mentioned industry standards, I'd imagine this is part of the reason why this hasn't been resolved yet. As far as professional and educational workspaces go, Macs are still the industry standard for graphic design (despite the fact that this hasn't been necessary for a few decades now). And I have yet to see a primary Mac display set up in portrait mode. Maybe if mainstream desktop workflows change, this issue might get resolved. But in the meantime this seems to remain a niche issue (hence only 36 votes in 2 years) and there are plenty of other players in the field that can fill that niche. @sjc presented some good alternatives. It's a shame because I really wanted Illustrator to be a one-stop solution for desktop and tablet vector design, but I found the tablet mode to be clunky even in landscape, so I'm not sure I'd even use Illustrator in portrait mode on my tablet if they offered it (other than in a tablet/multi-monitor desktop setup, and I grew impatient waiting for that functionality). This is all to say, if you're part of the niche, don't expect niche solutions from the industry standard, find another service that caters to your niche.

The solution is to use Inkscape, Corel Draw, Affinity Designer, or some other program, depending on what you are trying to accomplish.
Corel Draw is really great these days, actually!

While Adobe does have the "industry standard" illustration software (or did, it's rapidly losing market share because of tablets), I have found that specifically because of the issue in this thread, it's easier, better, and faster to just use a different tool that is designed specifically for what I want, such as Sketch, Sketchbook Pro, Inkscape, or Gravit.

There are no solutions. They may implement rotate canvas, which could help, but the UI would still be in landscape.

Are there any solutions so far? Just got surface pro and illustrator is absolutely unusable! Even a floating keyboard over illustrator window is not synchronised with illustrator window, if you want to lock the device in portrait mode you are not able to type anything in illustrator because click on the keyboard rotates screen one side, click on the canvas - another.. what a mess

Please fix!!

This basically makes Illustrator unusable when I'm working on a multi-monitor setup with my Surface in portrait next to my monitor. Please fix this!

What's even worse is that the program is able to be snapped to one half of the screen if you snap a different window first, essentially making it behave like a small portrait oriented touch mode. I just want that, but for the entire area of my screen.The loss of portrait touch mode is intensely frustrating.

This is the single most frustrating "feature" of this program, and it is the TOP reason that I close the program ever. And, it's why I'm hunting avidly for a tool that will replace Illustrator.

I've had it!

I don't get it... Every time when I think: yeah give it a tray, it looks like fun.... I'm ending up frustrated. No portrait mode, no working eraser function for my surface pen.

Just some love here and there and this mode could be a real dealbreaker!

@shauwn stone apparently the devs at Adobe don't want to draw in portait mode... and probably don't draw at all. Why they would have the feature available in a previous version and then deliberately remove it is absurd.

It's so embarrassing that this isn't a feature. Who the **** doesn't want portrait mode when drawing?

I also have this issue.
No matter what workspace I'm in, Illustrator won't allow the computer to rotate into portrait mode.
If by design, this is illogical. The user can always lock rotation in the device if they want to, but if it's locked in the program like this there's no way for users to force it back.