now I’m not so sure.
the problem files are all created with “curl … -o file.mp3” and regardless of what I do after ST saw them they cause the issue.
if I -o the files elsewhere and rsync them into the ST shared folder, it seems to work fine.
not sure in which direction to think next.
oh! of course! the colon!
it was was the cause of the problem. somehow android 10 is ok with it and 11 is not (on oneplus 8). haven’t noticed any difference in the output of mount on both devices but I have not yet looked diligently.
thank you very much for helping me see the obvious