Why does the BF sometimes not dump regression.diffs?

Hello,

I saw this failure after a recent commit (though the next build
succeeded, and I don't yet have any particular reason to believe that
the commits it blamed are at fault, we'll see...):

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=gombessa&dt=2021-03-01%2004%3A58%3A17

Strangely, it didn't spit out the regression.diffs so I don't know
anything more than "commit_ts failed". I emailed the owner asking for
that log but I see now it's too late. I wonder if there is some
scripting in there that doesn't work correctly on OpenBSD for whatever
reason... I know that we *sometimes* get regression.diffs from
failures in here, but I haven't tried to figure out the pattern (which
animals etc). Here's a similar failure that does show the .diffs I
wish I could see, same BF client version (11), on Windows:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2020-07-13%2000:47:47