I tested with a downloaded copy of 4.24. It looks like this would also happen in 4.25 though.
UAnimInstance::ParallelUpdateAnimation() does this:
// Tick asset players for this and any linked instances we have
for(UAnimInstance* LinkedInstance : GetSkelMeshComponent()->GetLinkedAnimInstances())}}
That gets called for both the regular anim instance and the post process anim instance.
A minimal repro sample project was provided on this UDN: https://udn.unrealengine.com/questions/569091/animation-playrate-in-linkedanimgraph.html
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-92007 in the post.