I can get the issue to occur with just a slight ascend, not forward or backward input. It does seem to required user input on the sticks, so I think it is software related to trying to preemptive move gimbal for steady shot. To test the theory or it being related to exceeding the stops you can try this:
1. Power up mavic mini and controller
2. Do not lift off, just point the gimbal straight down.
3. Move the mavic mini around manually (holding it, props not spinning). I can not get the gimbal to move off the -90 setting. If I exceed the angles it can handle the picture tilts but when I return it to normal level flight it goes back to -90.