When we unlink the layer, we call UAnimInstance::PerformLinkedLayerOverlayOperation
In that function, ClassToSet is null, so we loop over the layer nodes and call SetLinkedLayerInstance(this, nullptr)
FAnimNode_LinkedAnimLayer::InitializeSelfLayer is then called for each layer, which:
For any subsequent layers, the target anim instance is now deleted so we can't read the blend out settings.
When the layer is unlinked, the pose will snap instead of blending for one of the layers.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-150466 in the post.