Camera processing time / buffer screws up (delays) the camera initiating exposure starts quite frequently.
Most users incorrectly discount camera buffering / processing time as inconsequential.
Have you or one of your friends ever said something to the effect of:
"My camera can shoot 1 billion frames per second in rapid fire mode. I have a Class 50 card that can write 4Tb/s. It isn't my camera. Period. Don't imply my camera could could ever miss a shot. I just spend $XX thousand on it and it is amazing. If you do imply my camera can't shoot 200megapixel at 88 frames per second for 2 years straight, I will get angry."
Yes, yes, there are great cameras and they are getting better all the time. Still, watch out as camera processing, and buffering is still an issue with most, if not all, modern cameras when long exposures are involved.
The reason your camera's key light stays on for a few seconds longer after a long exposure isn't the write time to the card, its the camera, trying to make sense of a very long sampling of data. That takes processing time.
Do your own tests. Cover your eyepiece, put on a lens cap, manually set your exposure to 30 seconds, put your camera into rapid fire mode and hold down the trigger and wait for 5 minutes.
After a few shots, the recycling time from shutter close to shutter open will change . . .and increase.
Prove us wrong for your camera model. We need your help!