I had a similar problem where the rendered video would show angles from cameras that weren’t used in my sequence. Now, I’m new to UE so this could easily be something I did wrong but incase this helps anyone this was my issue and the solution:
I used the pre-built Polar Sci-Fi Facility from Marketplace. I stripped out the lighting and re-lit the environment myself, created a new sequence, added a little character animation and multiple cameras that I created. When I played the the sequence in sequencer it works fine but after I rendered it out new camera angles and a fade appeared in the exported video.
I tried all the tips people suggested here but none of them worked for my situation. The solution in the end was deleting all the cameras that were already in-place in the Polar Sci-Fi Facility (even though none were used in my sequence) and I deleted the already in place Level Sequence ‘Polar_Facility_Sequence_Daytime’.