Hi Rob,
I think this may have been fixed in the latest version of Moodle 3.3.2
I was reading the bug fixing log for this latest upgrade and there was one tracker issue MDL-60317 which I thought may be related. The only thing is that it was caused by a regression in a previous fix.
if after reading that tracker you feel it is not the same problem you are faced with then it is defiantly worth your while reporting this as a new bug in Moodle Tracker.
Hope this helps?
Mary