Hola Dan,
Noticed that Tamas Bunth checked in two patchs today regarding MySQL
foreign keys, it is marked as fixing issues in
https://bugs.documentfoundation.org/show_bug.cgi?id=122437
I wonder if it might also address the problem here.
I would give it a try with the latest 6.3 daily build in a day or two and
see if that does it.
Best wishes,
Drew