I believe the spec is limited to 16MB artwork, but that's per PICTURE block. I've just added a bunch of artwork I downloaded from MusicBrainz to a FLAC in foobar and it did complain when I tried to add one that was greater than 16MB, but the others have saved fine, the final file being over 92MB (foobar seems to limit to Front, Back, Artist, Disc and Icon).
Look at the section '9.8. Picture' in the latest draft here.

I also converted that FLAC using foobar and it was written with libFLAC 1.4.2 20221022 which is the latest.

I assume you're storing these only in track 1 are you? :-)