This may or may not be a bug; it was suspicious so I am
reporting it.
I ran the script

#!/bin/sh -x
for i in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
19 ; do #

    /usr/sbin/sendmail support-issue@foo.com <<EOF
From: Will Partain <partain@foo.com>
To: support-issue@foo.com
Subject: [issue$i] [assignedto=partain]
EOF
done

to assign a whole bunch of issues.  It succeeded on all
but three
issues, where it apparently did nothing.  I ran those
separately later,
and they changed assignedto as expected.

"Silently not working" isn't really desirable (if
that's what it's doing).  It should be easy to test to
your satisfaction.  Keep up the good work!

Will