Since the latest update to Thunderbird (115.4.1) the ability to mark messages as "read" does not seem to be working as before. Normally, I would expect emails to be automatically marked as "read" after opening them for several seconds; in other words, after actually reading them. Now, it does not appear to be the case - a large fraction of emails no longer are marked as "read", regardless of how long they are opened/viewed for, regardless of how many times one opens these specific emails. The only way to mark these as "read" seems to be to manually force the client to mark them via right click or some other menu.

TL;DR: Emails are no longer guaranteed to be marked "read", even after opening multiple times. Only occurs since latest update.