I think it’s probably more a situation where they are not a good fit for each other anymore. The D&D license has value and Hasbro rightly wants to capitalize on that. Larian is a hot commodity right now and they don’t need to borrow the credibility that comes with a big license like D&D. There’s also a timing issue. BG4 is unnecessary when BG3 will continue to sell for years to come. Larian will put out at least a couple more games before BG4 makes sense.
Larian is in a position where they can make whatever game they want and it will sell like hotcakes. Why the hell would they want to pay enormous royalties again when they can bring the writing in house? Sure, Hasbro could reduce their fee, but they can’t reduce it to the point where it’s worthwhile for both them and Larian.
If I’m running Larian, there’s no way I’m making another D&D game. The lore is great, but the rule set sucks. There are better systems in the tabletop space and there’s no reason to even be limited to that after you’ve already made the decision to not make D&D. Wizards isn’t exactly a paragon of reliability and stability either so there’s risk there. Not to mention, it was Larian who helped pull Hasbro’s asses out of the fire. They were facing massive backlash from their core customers until a kick ass movie and BG3 made everyone forget about it.
In short, Larian is riding high and Hasbro is not. There’s a lot more money for Larian doing something else and probably good money for Hasbro licensing to another developer.