'comm' and 'sort' incompatible?

[Dave Close]

Given two plain text files, I get this result:

$ comm -3 <(sort file1) <(sort file2) >/dev/null
  comm: file 1 is not in sorted order
  comm: file 2 is not in sorted order
  comm: input is not in sorted order

But if I make a slight modification, the result is:

$ LANG=C comm -3 <(sort file1) <(sort file2) >/dev/null

This appears to be due to both of the files containing a few non-ASCII
characters (UTF-8). Either 'comm' or 'sort' (or both) don't seem to
handle them properly.

Fedora 41 x86_64, but I've seen the same thing previously. My searches
don't reveal any previous complaints.

Trying the same thing on my Raspberry Pi (bookworm) does not exhibit
the problem.