Perhaps change create_figure to no longer include a \mu in the text in order to hopefully keep the baseline more robust against texlive version changes?
(If you do so I would also replace the \usepackage{sfmath} in test_rcupdate by perhaps something slightly more common, as I'm not sure sfmath is installed by default (I ran into it not being installed a couple of times)).

That's also a possibility; I wasn't sure if we used that specific text/package for any good testing reason.

It appears that replacing with Unicode Greek alpha does not work directly, as that glyph does not seem to be available in all the cases that the tests run. Looking at the text again, it's actually 'MICRO SIGN' (U+00B5), not the Greek letter mu. Trying the latter actually ends up with an empty glyph as well. So this is specifically a character from the Latin-1 Supplement block, which ü and ° already cover. We could change it to 'PLUS-MINUS SIGN' (which would probably not italicize) or perhaps remove entirely.

I've changed it to Section Sign to keep a similar text dimension, and confirmed that it works on Fedora 38 (texlive 2022) and Ubuntu 22.04 (texlive 2021).