Currently running Anki 2.1.49 on Windows 10. I have been trying to adjust the user interface size in Tools - Preferences to 90%. Every time I leave however the value resets to 100%. I tried changing some other preferences such as Night mode on/off and these seem to work.
I don’t think you can set the user interface size to 90%, that’s why it is not accepted. You can only increase the value. Try setting 110%, it should work.