Windows shows the thumbnails in Explorer using what is called a "Thumbnail provider". Windows 10 ships with such a provider for FLACs.
What I was trying to tell you is that there are other providers (applications) to do the same thing and, for all you know, you may actually be running some non-default provider which got installed with some other application like dbPowerAmp.
There's no easy way to tell, as the functionality is identical from you PoV. Maybe it stopped working correctly precisely because it's no longer the default Win10 provider that is generating the thumbnails. There are tools to check what is the current provider (NirSoft has one, I think).
So, I suggested you try Icaros, which is a well known thumbnail provider for FLAC and others, probably better than the default Win10 one. It works just as the default - thumbnails should become visible in Explorer.
PS: re-reading your post, it sounds like dbPowerAmp is in control of the thumbnails. So don't be too quick in blaming Microsoft.
PS2: I note that you are not SixtusO
My solution is for SixtusO problem, it sounds like you have your own workflow already. Stop hijacking the thread!