There's a bug in the anim bp compiler which results in split pins on animation nodes not being merged properly back onto their parent pin during compilation. The result of this is that any input to these pins ends up getting ignored at runtime. The code responsible is in FAnimBlueprintCompilerContext::MergeUbergraphPagesIn()
This method only gets run on the top level graph within the anim blueprint. Any sub-graphs do not get fixed up.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-110865 in the post.