Turns out he had legitimate reasons to call off his engagement to her like
A) he had to wed the dragon priestess and have an legitimate heir with her but fl refusing even before marriage
And saying that she would only be a wife in name and he should get a concubine is a receipt for trouble
Like the prince would need absolute backing from his faction after ascending the throne and you are telling me that he is basically throwing away support
cause if he got a concubine who is to say that gurl isnt going to get piss off at having to do all the queens work and still not having the title meanwhile the queen is in the dragon cage
(All if this is hypothetically and from the position the prince was in at the start if the story he wasnt aware of the dragon dying or that fl had agreed to be the dragons bonded pair)
As well as some noble holding a grudge over this whole thing mad at the fact that their daughters would have deserved that position more
Then there is
B) he believed the investigation from the dragon department that fls dragonstone lost its light and that the sister was the new / next dragon priestess he wasnt aware of the collusion between the sister and his brother (an alliance of evil siblings lol)
(Goes to show that if u want something done right do it yourself)