Which REST client are you using? Which language? I've used PI Web API against 10 different platforms and I have never faced this issue.
I think the easiest way is to call a PI Web API Batch request (which is a HTTP POST request) and you can define an inner HTTP PATCH request.
Let me know if this works for you!
Thanks, Marcos! Although the batch request was a bit more complicated to implement, it did the trick!