grep -v -U 'pattern' filename matches files that do have 'pattern' as well as ones that don't

[Paul Pomes DVM]

Summary: grep -v -U 'pattern' filename matches files that do have
'pattern' as well as ones that don't

I'm trying to see which of my *.flac and *.mp3 music files already
have the replaygain_album_gain tag.

In a directory where mp3tag confirms the presence of the tag in all music files,

grep -v -U replaygain_album_gain *flac                    prints

Binary file 01 Panatela.flac matches
Binary file 02 Lament For Linda.flac matches
Binary file 03 Misery, Stay 'Way From My Door.flac matches
Binary file 04 In A Misty Mood.flac matches
Binary file 05 Catty'corner.flac matches
Binary file 06 The Thirteenth Instant.flac matches
Binary file 07 The Magpie.flac matches
Binary file 08 Blues For Indian Jim.flac matches
Binary file 09 The Devil And The Stoker.flac matches
Binary file 10 The Swing Machine.flac matches
Binary file 11 Summer Nights.flac matches
Binary file 12 Johnny On The Spot.flac matches

which is the same output produced as expected by
grep -U replaygain_album_gain *flac

OTOH searching a file *without* the string works as expected with -v
and without:
grep -v -U replaygain_album_gain *jpg
Binary file art-back.jpg matches
Binary file art-booklet01.jpg matches
Binary file art-booklet02.jpg matches
Binary file art-booklet03.jpg matches
Binary file art-booklet04.jpg matches
Binary file art-booklet05.jpg matches
Binary file art-booklet06.jpg matches
Binary file art-booklet07.jpg matches
Binary file art-booklet08.jpg matches
Binary file art-booklet09.jpg matches
Binary file art-booklet10.jpg matches
Binary file art-booklet11.jpg matches
Binary file folder.jpg matches

and
grep -U replaygain_album_gain *jpg             produces nothing.


Thanks in advance.

/pbp
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