Why would VS2019 (with VisualGdb) refuse to show added files the solution explorer?
I found some workaround, i will post as an answer. But i'm still quite interested in the explanation of the underlying problem.
Additional info: opening the project file (.vcxproj) shows that the files are actually added to the project, only they are "invisible" in the project explorer.
I'm not sure, if this issue is VS-related or VisualGDB-related, or even layer-8-related, but hopefully this answer will help some people anyway.
So after trying many things not leading to success, i found this workaround:
In the project file (.vcxproj), i actually found entries for theses "invisible" files, and after deleting them and re-opening VS, i added these files again in the soluton explorer. Only this time, they correctly showed up in the solution explorer.
Weird: a compare tool showed not the slightest change in the project file, so there got to be some underlying issue.