To clarify the response in this reddit thread, I believe the Beast Heart is only convertible to Raid Seals by the random merchants that show up in strongholds. I had one show up that gives 10 Raid Seals per Beast Heart.
As far as I can tell, it's not 10 raid seals but 9-12. So if you aren't in a rush, wait for a day with a 12 offer. Same goes for Unidentified Relic and Bandit's Treasure