This issue was caused because the RadeonSettings.exe app uses (wait for it!) TAHOMA as it's font - a font that hasn't been a system-default font in Windows since Vista. And its hard coded (I assume since as we develop software here, we don't hard code that : you look for the default system font and use whatever it is where it should be that, and if other fonts are preferred for other parts of the UI you look if it's there and use that or else have a fallback or install what's needed. Basic stuff.)
We use font substitution for Tahoma to replace with Consolas. No Windows anything uses it and one legacy app uses it for text in a place where numbers are prevalent and O vs 0 matter, etc. Never had an issue for over 12 years of doing so on all our machines.
Worse though is the MASSIVE amounts of buffer overflows the app produces - 195 just to load the app. These are really, really not a good thing! Who codes this stuff? Just running System Internals Process Monitor (so not like developers doing their own testing, which clearly seems to have been absent) raises an alarm. How many are exploitable?!?
This was NEVER going to be an Apple support issue. That AMD rakes in massive cash from Apple but punts off any support is pretty weak.
Anyway, thanks for your persistence even if the assumption is some user initiated thing, which in forums I guess assumption is "not real sharp knives" when it comes to users seeking answers.
Hope someone over at AMD sees this.
Since Apple Support is directing you to AMD. I haven't posted the link to AMD Support. So here AMD Service Request (AMD SUPPORT): https://www.amd.com/en/support/contact-email-form
Maybe they might be able to show you how to manually change the font or suggest a work around.
Sorry I couldn't be much of help. But generally Bootcamp issues are referred to Apple Support.
By the way, Both Moderators of AMD Forums are AMD Employees. On rare occasions they will open a ticket to the appropriate AMD Dept if they feel it is important enough. But their main responsibility is running AMD Forums.
Except that it was never a Bootcamp issue - it was an application written by AMD for it's GPU running on Windows. Bootcamp is just to allow running Windows and macOS on one machine - an iMac which is just a PC at heart (literally as in Intel processor etc.) If I had not mentioned Bootcamp probably different. It was also not a driver issue: it works.
Anyway, fixed it by removing the Tahoma to Consolas replacement, using the app to do what I needed, then restoring the Tahoma replacement.