I have some upside down sequences a while ago and the other day had a 90 degree one which was odd.
The upside down ones I had the phone the other way, Mapillary displayed the right way on the screen but the phones top bar was upside down. Since then I use the phone the opposite way and everything appears upright.
The 90 degree one I activated the camera before putting it in the mount while it was in my hand so it must have set the orientation different and I had no idea until I got home and started uploading.
I now make sure that my phone is securely in the mount before I load the camera and usually before I load Mapillary too just to be sure it knows the correct orientation.
I would be nice to batch run sequences on your own uploads to fix or rotate images either in browser or on the device before uploading..