Sounds like it is the dynamic iris. The response is slow and distracting. That's my one negative with this projector. The iris closes and lets less light output to get deeper blacks. I've tried setting the dynamic iris to medium, low and off but can't tolerate the raised black level so I leave it at high.I recently purchased the HT5550 and have the latest firmware 1.03.
Blu Ray Player: Sony UBP-X800M2
When I’m watching 4K movies, I’ve noticed between scenes that the colors will randomly adjust, get a bit brighter than adjust back. I’ve noticed it more frequently in dark scenes. I never noticed this on my old BenQ W1070.
Any tips would help out.