There must be some rule that historical yuri can only have bitter-sweet endings.
There isn’t a specific rule, but there was a common trope in yuri literature, maybe around the 1960s in Japan, where yuri stories often had to end tragically. Authors weren’t allowed, or at least weren’t encouraged, to write openly queer happy endings. My information is a bit fuzzy, but I remember watching a few video essays and reading some articles about it. If I find them, I’ll link them. :)
Outside of Japan as well there's a lot of things with same sex relationships where there was push back against them being allowed a happy end
I wonder if it's stuff like this, that give the religious types the basis for their argument, that queer people live in depression and sorrow. I've heard people use this argument as an excuse to force queer people to live a straight life under god.
People that oppress others love to argue that if the people they're oppressing just listened to "what's best for them" they'll lead a happier life. Religious people/groups have a lot of tactics in that regard, and it's gross all around.