I noticed that saving the level also reset the camera Filmback to the “First saved setting ever”
repro steps,
1- create new CineCamera in Sequencer, set the filmback desired,
2- save sequencer, close and reopen it.
3- Now switch the filmback setting to another camera type.
4- Without closing sequencer and with camera highlighted.
5- In the main editor interface; Click on the top left FloppyDisk save icon and look at the detail of your camera change back to the original saved setting.
I get 100% repro rate at work and at home in different projects, even from a brand new project this happens.