Thanks to some great feedback I have identified an area of code I had not previously tested for this error. It also gives all users a temporary solution to at least some instances of this problem.
It is now clear that when adding a fact or event to either an individual or a family record there is a different response depending whether you select the fact or event from the “Select” drop-down, or from the “quick-list” of options just below that.
The quick-list produces the issue as described in this thread, the drop-down select list does not. So, temporarily, I suggest you do not use the quick-list.