I will add a final follow up. This issue was resolved, and I’m not sure what fixed the problem. It could have been a missing codec which became available to the Wayland session after a reboot… I did not make any significant changes after our last discussion.
It could also have been a software/system update, but comparing the current version of software with the ones above does not show a discrepancy. (same KDE version, kernel, QT version, etc).