During interval timer shooting, NIKON D5600 overexposes photos randomly

I am having an issue with my camera. At random, it will overexpose photos during interval timer shooting. It’s generally fine at the beginning, but as time goes on, there are more and more overexposed shots. The time on those shots is the same as on the ones that are properly exposed.

I am thinking that either the aperture is not being set properly on the lens, or the shutter is broken.

How can I diagnose this so that I can know if I need to get anything fixed? (It never happened when I was taking pictures normally).

Here’s my last timelapse, you can see the overexposed shots: