More sleuthing has uncovered something. Puddletag shows various files with Playcounts with 2 values e.g. 0\0, 0\258, up to 0\65335.
I tried to delete those screwy counts in puddletag by overwriting them with 0 but it doesn't seem to have taken. Or they've regenerated.
Checking in kid3 it turns out that one of those files, with 0\258 has
FMPS_PLAYCOUNT=0
FMPS_Playcount=258.
If I listen to it once in strawberry:
FMPS_PLAYCOUNT=0,
FMPS_Playcount=259.
So strawberry increments FMPS_Playcount but not FMPS_PLAYCOUNT.
I wonder what the official/correct tag name is?
https://www.exiftool.org/TagNames/ID3.html this says it's "PCNT" for id3 v2.4
According to https://github.com/strawberrymusicplayer/strawberry/issues/1074 , FMPS_Playcount (Strawberry spelling) is CORRECT, so logically something else has been using FMPS_PLAYCOUNT.
But why do they (second entry, FMPS_Playcount) go up to 65335?
And does overwriting them with 0 in puddletag not work because it's displaying 2 variants of the same tag?
In puddletag I can't create separate columns for the 2 spellings because it automatically uppercases them but displays both.