HTML errors in calendar

I use the calender on microsoft desktop and apple ipads.

I appear to be getting html errors on calender events on my ipads. It seems to happen when i go back an edit a calendar entry (after it is originally created) on my desktop. (which was never an issue before)

See below before and after picture of what happens. It has only started happening in the last 6 months before that there was no problem. I have completed all updates on all devices and it did not make a difference.