That blank issue could have caused by update that I did revert wine version to 3 but later reverted back to 6, I was testing audio issue which wine 3 fixes it but unfortunately it introduce other issue.
Regarding adding files to playlist I’ve added new changes to f2k snap which should now show Enqueue in foobar2000 in right click on files/dirs using openwith. So using foobar2000 only with parameter or with openwith will replace current playlist.
if you still want to add files using terminal then do this
foobar2000 /add ~/Music/some-file.mp3
foobar2000 /add ~/Music/some-dir