Depending on whether you're logged into Adobe account or not, there are 2 places where Auto-fill entries are saved:

1) If you're logged in, the setting is stored in the cloud, and you use the web interface to change the preference.

2) If you are NOT logged in when using Acrobat Reader, the Auto-fill entries are saved LOCALLY in %USERPROFILE%\AppData\Roaming\Adobe\Acrobat\DC\Security and is controlled by Edit -> Preferences -> Forms -> Auto-Complete

HOWEVER, Auto-Complete is usually OFF by default, hence the reason for this bug - Acrobat Reader is STILL saving Auto-fill entries on the local machine if the user is not signed into Acrobat account.