If you press Restore Defaults in the Add-On configuration, the changes will (visually) only have an effect once you mark the text inside the configuration.
This could be confusing/misleading. Same behaviour in Anki 2.1.26, so this is nothing new probably.
Anki 2.1.28 (f44597bb) Python 3.8.0 Qt 5.15.0 PyQt 5.15.0
Platform: Mac 10.15.4
Flags: frz=True ao=True sv=2
Add-ons, last update check: 2020-06-23 07:48:57
Reproducing steps as video in
https://anki.tenderapp.com/discussions/beta-testing/2017-httpsforumsankiwebnettrestore-defaults-config-not-visually-updated420?unresolve=true
(forums.ankiweb.net doesn’t have *.mov allowed as upload)