Hi Michael,
I just filed a bug report
http://lilypond.1069038.n5.nabble.com/Wrongly-read-property-with-MetronomeMark-td237659.html
2c2908c905ba822ef656b06b1cc4f0ca33960c9c is the first bad commit
commit 2c2908c905ba822ef656b06b1cc4f0ca33960c9c
Author: Malte Meyn <lilypond@maltemeyn.de>
Date: Sun Sep 29 10:10:35 2019 +0200
That does not make sense to me. 2.20.0 was released on March 1, 2020 and
Harm reported that 2.20.0 is fine. I think bisecting must start with
2.20.0., then.
It's well possible that I got this all wrong, but I was under the
impression that, as not all changes added during the 2.19.xx cycle
were cherry-picked into 2.20, it's quite possible for a commit that
was added to master before the release of 2.20.0 to only end up in
2.21.xx.