Well, this is a strange one.
So far this seems to be affecting only one user. Whenever a task is assigned to him, within 10-20 minutes, the system resaves a new version of the task, but with all the HTML formatting stripped form the task description field. This means not only fonts & colours are lost, but line breaks too.
In the version history, I can see that this new "version" is being saved by the user to which it was assigned, but it happens even when his computer isn't even on. At no time did he access SP via his phone, so we can rule that out as well.
This has been happening since we migrated to SP2013 Online, about 2 weeks ago. Any ideas on where to look? I'm stumped.