The 4 cases are not the same.
Bulgaria and Greece were former King and Queen (consort). AFAIK, they have precedente over Crown Princes. Their seating was ok. After reigning monarchies and Bulgaria before Greece.
With Romania and Serbia, their precedence has no explanation. Their place is after representatives of reigning monarchies.
Interesting to see that the heads of the non-ruling dynasties from Bulgaria, Romania, Serbia and Greece were given precedence over the heirs to the thrones of Japan, Denmark and Norway.
428