carmine peak
So, after seeing how the winter season is bugged out in general, and the fact I 10 star'd my romanceable character (Mark) during winter, I tested out the theory of mine.
The theory that if you ten star'd them during winter, it will bug out and you won't recieve the letter.
I speed run the game (with mod helps ofc) with the goal of having 10 stars with Mark before Winter.
And somehow Lo and behold, I received a letter days after getting him to 10 stars (and leveling my house to 2)
I don't know if this will work with characters who also have this issues, but try maxing the romance before winter. It may be the temporary work around in my theory