version: 2.92.0 Alpha, branch: master, commit date: 2020-12-04 20:46, hash: ea37e4ea5a71, type: Release
build date: 2020-12-05, 00:06:39
Not working in 2.91 & tested not working in 2.92.0
Legacy undo handles this properly.
Short description of error
If you change the dimensions of an object with a mirror modifier using the transform box, then undo that change, the dimensions will display incorrectly and if you will be unable to change them correctly again.
Exact steps for others to reproduce the error
1 - Enter edit mode and edit the default cube to move one face to 0 degrees (for example move the top face from 0,0,1 to 0,0,0)
2 - Add a mirror modifier to the cube, mirroring the axis of the face you moved (in this example, select z). The cube should now appear it's original 2x2x2 size.
3 - In Object mode, use the sidebar "Item" tab to enter 1 in the "Dimensions" value for the axis with the mirror modifier. (in our case z) The cube should change size.
4 - Undo the change using Ctrl-Z or Edit-Undo
The "Dimensions" section will still show the 1 that you entered. If you try to enter "2" to get it correct again, it will act like you entered "4" as it seems to apply that number to the object before the mirror.
Further, if you have multiple axes setup like this (as in the attached example file), when udoing all the axes will change to 1, even if you only edited 1 of them.
Switching in and out of Edit mode on the object will reset the dimensions so they show correctly.