This is on Fedora 34 server.

$ uname -a
Linux fedora 5.11.16-300.fc34.x86_64 #1 SMP Wed Apr 21 13:18:33 UTC 2021 
x86_64 x86_64 x86_64 GNU/Linux

$ comm --version
comm (GNU coreutils) 8.32

Consider the 2 attached files f1.txt and f2.txt. They're already sorted.

$ comm -12 f1.txt f2.txt
1O55
1O56

This means that 1055 and 1056 are in both files. However, this isn't
true.

$ fgrep 1055 f1.txt f2.txt
f1.txt:1055
$ fgrep 1056 f1.txt f2.txt
f1.txt:1056

Note that these lines are only in f1.txt. This looks like
a bug.

Please advise.

Cordially,
Jon Forrest