Yeah, level 12 is a critical level because of this, which is exactly why having it as the final level is so very problematic for multiclassing.
Final ... for now. I'm personally hoping BG3 is our BG1 equivalent and that we'll get the chance to continue developing our characters in expansions/DLC, or even a sequel.
Personally I wouldn't want to rush the story of my PCs' adventuring career for the sake of getting to higher levels in one game, and while I'm not a 5e expert I can see that level 12 makes a lot of sense as a stopping point for single class characters. Not so much for multi-classing, I understand, but that's the choice folk will have to make about the costs/benefits of multi-classing their characters in a low-mid level adventure which seems fair enough.
