The NVidia decoder implementation by Milestone has some load balancer logic and it will stop using the Nvidia GPU if the memory utilization is above 80 percent. This is a hardcoded limit that you cannot change, the limit is based on experience while developing and testing at Milestone. I guess this fit perfectly with your observations.
Thank you for your feedback. I consulted an expert in Milestone Development and they will take it consideration but we cannot say when (or even if) this will be developed as there can also be larger memory fragmentation even if there is a lot of memory available.