This was tested in El Capitan and in a colleague's High Sierra, in the standard Terminal (bash).
user@hostname ~ $ man ls | grep "BU" BUGS user@hostname ~ $ man ls | grep "BUG" user@hostname ~ $ user@hostname ~ $ man ls | grep "IEEE" files in order to be compatible with the IEEE Std 1003.2 (``POSIX.2'') The ls utility conforms to IEEE Std 1003.1-2001 (``POSIX.1'').
To clarify: "BUGS" is a section title in that (and various other) manpages. For section titles, grepping only seems to work for the first 2 characters; this is consistent across a few different section titles we tried. For the rest of the content,
grep seems to work as expected.
I ssh'd into a non-BSD flavored Linux box (Amazon Linux) and it does not appear to exhibit the same behavior.
What's going on here?