I'm having the same issue. Email I sent to support:
Hi,
For some reason bosses (vindicators, elder guardians, etc) do not seem to be spawning in my world. I am aware that they don't spawn ever again if you load their containing chunk on peaceful, but this is even happening in areas where we've never loaded peaceful.
Do you know what might be happening?
I know that they despawn during peaceful, but this was happening in areas where I never had peaceful on when loading chunks. Additionally, I'm seeing the same issue with bosses in woodland mansions and I know for sure that I didn't load that in peaceful because it was in a direction I had never gone before.