Maybe try
USkeletalMeshComponent::GetCurrentClothingData_GameThread
There is a part in the doc that states
Get the current simulation data map for the clothing on this component. For use on the game thread and only valid if bWaitForParallelClothTask is true.
perhaps the bWaitForParallelClothTask is crucial? Maybe you are getting the clothsim before the task is complete, so from 1 frame back?