It turns out it appears to be a program cache issue, probably do to the upgrade.
Steps that led me to this problem:
I was on 188.8.131.52 and had the "Light" setting on (if that possibly has something to do with it).
I saw the upgrade option to 184.108.40.206 and upgraded without uninstalling the previous version.
I couldn't use the polygon, magnetic or mode options (they simply wouldn't select, and the modes looked disabled)
I uninstalled (I believe restarted) and then installed 220.127.116.11 again, but the problem persisted.
Steps that resolved the issue:
I uninstalled all my Affinity programs
Deleted Affinity directories in the registry
Deleted Affinity directors in "App Data" (including the temp setup files)
Installed 18.104.22.168 of both Photo & Designer
The UI is back to "Dark", but everything works properly, even if I switch it to the "Light" UI.
Based on that, I think there was a caching issue from either the registry or in App Data that caused the bug as part of upgrading.
But now I'm up and running again like new, with everything working well!