Maintain width and Height Proportions bug
This has been a bug for at least a version or two: draw a shape, go to transform it in the Properties panel, and even though the "Maintain width and Height Properties" icon indicates "not constrained", adjusting one of the dimensions adjusts the other proportionally. This seems to happen only in a fresh file (perhaps fresh launch of Illustrator), and once I click and unclick the Constrain button once, it doesn't recur in that session.
https://www.dropbox.com/s/u0gqadn2lezzq7n/Constraint%20bug.mov?dl=0
So in the screen grab above, I draw a box freehand, then in the Properties panel, begin adjusting the height with my arrow keys. You can see the icon to the right indicating width and height are not locked together, but still, as I adjust the height, the width changes in proportion.
Note also that once I click that icon to specify "Constrain," then unclick it back again, now I can adjust the two parameters independently. And from then on in the file, the bug doesn't recur.

I am happy to share that we have fixed this bug in our latest release – 26.1.0 which is available worldwide now.
Going forward, our goal is to fix as many top User-Voice bugs as possible and as frequently as possible. Given the nature of the fixes, some of the bugs will take a longer time to fix, but we are on it.
You can update to the latest release using Creative Cloud desktop App: https://helpx.adobe.com/in/creative-cloud/help/creative-cloud-updates.html
Thank you for all the feedback. Keep it coming!
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Anonymous commented
Have the same problem - just found out it screwed one of my jobs in print!
1. draw a rectangle
2. resize one side in properties panel (constrain icon is off)
3. it changes both dimensions as if constrain is enabledI consider this a major bug and obviously it created a problem for me when I did not notice it during size adjustment. Luckily it was only a small job - I just fear to imagine if I screwed up something bigger because of this Illustrator bug. It seems interesting that more people are not talking about this.
I thought I'm paying for a professional software.