I am using Microsoft 365 which is accessed via multiple devices (but one user). There are occasions, when suddenly a large number (last night was 60) of messages that had previously been read (and marked as read on OWA, Outlook or iphone) that became unread.
There is no link between the messages. Some have been replied to; they have been read on different devices. They are marked as unread on all devices and you have to trawl through the messages to mark them as read once again (which updates all methods).