I have a suspicion that something went
sideways with a W10 update and was (silently?) fixed (W10 does not give you control of updates).
If you were unfortunate to have done an install during the sad window, file associations were
tampered with
A number of recently updated Programs (Calibre, Sigil, NP++) all had their long standing default associations messed with. The most obvious artifact is the Icons in Explorer are not as expected)