Have you tried reproducing it in a fresh project? As a test I just tried and am not able to reproduce it.
The BehaviorTree component definitely exists within the runtime namespace. There's the BehaviorTree.cs MonoBehavior that you're able to check to verify.
AssemblyDefinitions seem to have something to do with it. In a clear project, the BehaviorTree does show up. If I try to access it from a script under an AsmDef, I see a number of classes but not the BehaviorTree.