Nikon USA issued a service advisory for the reflection/flare issue on the D750 camera. To check if your camera is affected and needs to be repaired, you can enter your serial number on this website (here is the EU link). A black dot inside the tripod socket on the bottom of the camera will indicated if the D750 cameras has already been serviced.
My guess is that this service advisory delayed the new firmware update program that was supposed to be announced yesterday.
Full text of the Nikon D750 service advisory:
