I'm sure he'd consider it unlucky considering his son is a mothercon. But the king does say 'every few decades' that's a small enough time period for literally EVERY king to have at least one child with that trait. Its entirely possible that its more unlikely to NOT have at least one child who's like that, and could be a cause for their family having so few children to reduce those odds. After all they have 2 sons and no daughters seemingly, they might have been worried about having a second son with the trait and turning their kingdom into super game of thrones if the two get interested in political battles against each other.
When I read the novel, I found this 'logic' to explain the genius prince trope is so well done. I also like the unusual monarch loving relationship like this. They usually non exist :'D
It's just the ending tho,,,
I feel conflicted to know that their son 'fated maiden' probably his own (mother, then shift to) sister???
The original source webnovel for this already ended a while ago.
And yes the manga adaptation will most likely also be ending soon as there's really only some extra bits following this. They're trying to slowly cover the story as best as they can and I don't see any issues with that. The story is popular so it makes sense that the author and writer want to portray the story in the best way possible