I also faced such errors much time. Opened many YouTube videos to fix such kind of problems. However, now, I decided to get actual size measurement assistance from this online application ( https://ruler.onl/ ). It is not a perfect solution but can avoid beating the illustrator problems with smart solutions.

I am having this same issue on my Wacom DesignStudio Pro with no extra monitor attached. You select "Actual Size" and it just seems to zoom to fit whatever resolution my screen is at currently. I am using Camtasia to record and the record viewfinder is set perfectly to 1280x720 and so that is how i first noticed this. Artboard is set to 1280x720 and until cc23.01 Actual Size zoom worked just fine. Changing resolution on monitor does not correct this or affect this at all. I agree that AI seems to not be able to determine the correct resolution of my screen,

Actual size (Ctrl+1) in illustrator CC 2019 gives me a 1" square displayed as 1.125"

I used to be able to set my screen pixel density in preferences to get the on screen ruler to match up with a physical ruler held up next to the screen ruler, but the new automated thing where Illustrator is supposed to get that data from the OS doesn't seem to be working correctly in all cases. It did on my old monitor, which was a 27" dell. The new ones our company gave us yesterday are 32" LG monitors.

For me, this has always been the case. Never knew an illustrator version to show a true 'actual size' before... so this works for some? I usually enlarge about 125% approximately to get it close to actual size visually.
Nevertheless, I am using my primary monitor (I only have one on my laptop) and the actual size is not even close. Tried different monitor resolution settings to no avail.

I've noticed this in the 2019 last update as well. My 1/1 does not seem correct. I use to be able to Command 1 on a 11x17 spread that would sit within my 24" monitor window, now it is seemingly zoomed in.

Please consider making this work with multiple monitors. I don't know many people who use illustrator on a single monitor. Folks, a feature request has been submitted (link below), please upvote if you feel inclined.

https://illustrator.uservoice.com/forums/333657-illustrator-feature-requests/suggestions/35799382--actual-size-should-work-with-multiple-monitors

"Actual view feature only considers your primary monitor."

Wait, so the new true-size feature ONLY works at the ppi of your primary monitor and ONLY if you're not working in terms of pixels?

(see https://illustrator.uservoice.com/forums/601447-illustrator-bugs/suggestions/35736118-100-view-is-not-100-for-pixels-in-cc2019 for the second part)

So all print designers with multiple different monitors, and all screen designers period have been left out to dry... I expect many people won't even notice that their "true-sizes" aren't true at all. Too bad for them I guess...

(Sorry to be so snarky but I'm honestly flabbergasted Adobe thought, and still seems to think, this true-size feature is a good thing. Something like https://illustrator.uservoice.com/forums/333657-illustrator-feature-requests/suggestions/33253402-100-size-is-actually-actual-size would serve the purpose, imo.)

Can confirm that, for a 1 inch document at 100% measured on 4 monitors, only the one on the main monitor was the correct size. The other three were too large by at least 1/8 inch.

Maybe that has to do with the fact the other three monitors have lower ppi, but either way "actual size" is wrong.

Possibly related to https://illustrator.uservoice.com/forums/601447-illustrator-bugs/suggestions/35736118-100-view-is-not-100-for-pixels-in-cc2019

I'm having a similar issue since I updated to CC 2019. When I select Actual Size, the view is actually slightly bigger than the real actual size. To view the actual size, I need to manually set the zoom level to 66.51%.