See the green square? That's how the program shows you who is currently holding the Pokemon. Remember that OT changes between different generation. Something hatched in gen 7, its OT is gen 7, not any other generation. Even when it gets traded to someone else in gen 7, that other person is not the OT. So the Not OT handler gets updated, because it wouldn't currently be in the possession of the OT. It's the same thing if the gen 7 OT transfers it to their gen 8 game. The gen 8 OT is not the person who hatched the gen 7 egg, so the Not OT handler gets updated.
Re-set the Horsea (Right Click -> View and then Right Click -> Set), or click on the box labeled Latest (not OT) Handler to manually set the current person who is handling them, because they wouldn't currently be in the possession of the OT.