I havent had any experience in this but the styles which shows this small icon (Ab) is in themesource/datawidgets/web/_datagrid-filters.scss file.
there should be styles as per the screenshot below.
can you see if there is any issue there? may be during the migration this file got corrupted or might be totally missing(which is highly unlikely)
Also do 1 thing: try to clear your browser cache or open the mendix application in an Cognito or InPrivate session to see if it appears there. I suspect this could be css cache issue.
As first blunt solution you can try to overwrite (after creating a backup) your Themesource directory with the one in you test-app. Might help.
If not, see if you can spot the difference between your app and your test-app in the browsers' inspector. Should look like this:
This can help you get to the root of the problem.