With 18.1 update of Adobe illustrator, we have added an UI option under preferences > User interface section to either turn off scaling completely or allow users to snap to lower or higher supported scale factor. Since Ai only supports 100%, 150% and 200% scale factors, users can choose to snap to 100% or 150% when his/her OS ui scale is between 100% to 150%. E.g. for 125% users can select lower supported scale factor to snap Ai ui to 100% and higher supported scale factor to snap to 150%. Similarly for OS ui scale 175%, users can snap AI's ui scale to 150% or 200%.
If you have defined the environment variable UI_SCALE_FACTOR=1, Then this preference will not work.
Sanjay.