I did use my “revert form” every time before leaving that room you do the training in. I am running a lot of mods, but almost all are crafting and texture related. I have no followers with me.

Is there a console command I could use?

I don't get it. You train to be a Vampire Lord in the castle, but have to revert back to human form and can't be one while exploring and being in other parts of the castle?

I haven't tried leaving the room as a vampire lord. Lord Harkon tells you to revert form, so I do. Yet I get attacked by everyone in the castle once they detect me. I'm going to try using TCAI.

So now I've tried both leaving the room as a VL and using the toggle combat AI and neither really helped. They still attacked me as a vamp lord and if a use TCAI they would stop fighting me but would still be arrgo. I left the castle and returned first with the TCAI on and they attacked me again. So I try with it off. They didn't attack me be they still wouldn't talk or interact with me. Just a note Serana still fights on my side.

Fixed the problem. Someone in another forum said to make sure I was crouching and "hidden" when I first changed into the vamp lord. This worked and everything seems fine. Still not impressed with the bug though.