On the wiki, it says that the game rolls 2 1d50s and adds those to prestige, and then you get no adventure for 1-25, minor 26-50, etc. That matches my prior playthrough experience. But this playthrough seems to be different. I have prestige 36, and have been reloading and rerolling adventures. It is the second one I've gotten. I'm getting average adventures every single time. I've done it at least 30 times. But if the wiki is right, sometimes I should get a minor one, and sometimes I should get a major one, since I should sometimes roll outside the 51-75 range. Anybody know if they changed the way stronghold adventures are generated?

They have not. But reloading the way you do wouldn't change a thing, your only out would be to shift your prestige before the adventure turn hits. For more context: 

 

https://forums.obsidian.net/topic/85392-caed-nua-adventures-how-what-level-when/?do=findComment&comment=1832708

That link is very helpful, thank you. I didn't realize that the dice were rolled earlier than when the adventure is rolled.