CacheBones_AnyThread not called in a custom blend graph

I’m trying to implement a new blend node (derived from FAnimNode_Base), and it works correctly in some situations I have tested.

However, a colleague needs to use it in a “custom blend graph” on a transition, and it seems in this situation CacheBones_AnyThread is not getting called by the engine. As far as I can tell, this is the case for other engine blend nodes, when I place breakpoints they are never getting hit (as opposed to Evaluate_AnyThread which is called as it should).

I’m wondering if there is a bug in the engine, or maybe I’m missing something?