I have a large set of slides (in beamer) that I use for my lectures, making additions/modifications, removals for outdated information, changes in lecture boundaries, etc. each semester. After a recent update (71 packages) in TeX Live 2021 that I pulled using tlmgr a couple of days ago, I have started noticing that the following constructions:
\SI{2.3}{\micro\gram/day}
result in Unicode errors complaining about the $\mu$ that is supposed to be generated. The change in siunitx version is 60105 -> 60258 (from the log). I have used such constructions for years, without a problem. That suggests that either the issue is with siunitx, or one of the other packages that impact handling of Unicode (inputenc with the option utf8). I checked — inputenc was not updated two days ago.
Has something changed in the packages so as to produce the effect above?
Second thing I notice: the text "T@TS1" now appears on the first slide by itself (before the titlepage). While this is annoying (without being a showstopper), I would rather find out what is causing these problems without any intervention from me.
Both these issues occurred immediately after the update — I had not yet started making any changes to the previously correctly compiled markup. These two observations may or may not be related.
The second bug above is addressed by siunitx v3 in beamer adds extra slide containing "T@TS1" before the first frame. The first one — with \micro resulting in Unicode errors — is not. This is a new, yet unaddressed bug.
Best Answer
For the present, you can add
I will need to track down exactly where the issue arises before sending an update to CTAN.
I have logged as https://github.com/josephwright/siunitx/issues/535. This is basically a timing issue to do with changing the font, which interfered with detecting symbol changes. I have altered the relevant code and will update shortly.