GPUScene primitive validation is currently busted and asserts. It looks like there are two problems:
1) NaN's in the entries can cause comparison issues, so a MemCmp would be better.
2) There's a velocity bit that is not being updated properly.
Enable r.GPUScene.ValidatePrimitiveBuffer. Observe asserts.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-96219 in the post.