There was a power failure today. After the power came back and I rebooted the pi, I had issues serving the site.
For some reason, the default Ruby had reverted on reboot. rvm use 3.1.2 --default did not seem to have done the job. I tried a variant, rvm --default use 3.1.2, and then discovered that I have to run bash -l after reboot before I can run Ruby/RVM commands. I tried again, and this time successfully switched to 3.1.2, and then was able to serve the site with bundle exec jekyll serve --livereload.
A few more reboots today for a variety of reasons, and it seems I always need to set Ruby to 3.1.2 after reboot. I have not yet been able to figure out a way to make the default actually default.