I have got exactly the same issue in 4.27. Issue was not tested in 4.26 by me, but in 4.25 everything was working fine.
If i use custom anim node based on FAnimNode_AssetPlayerBase then FAnimInstanceProxy::GetRelevantAssetPlayerFromState() fails on getting state info, otherwise FAnimNode_SequencePlayer which is based on FAnimNode_AssetPlayerBase is working fine.