The issue is that it seems the use of @JoinTable is generally problematic with the TABLE_PER_CLASS inheritance strategy. I tried the example with other strategies and that worked, but using TABLE_PER_CLASS fails with multiple different errors.
I think I have a fix, but I need to do some more testing. This needs time to review as it touches some of the fundaments of the TABLE_PER_CLASS inheritance strategy so it might not happen for 5.2.9 yet but maybe only a subsequent release.