The Faceboook Live 360 feature is new and we still need to collect more usage examples. Thank you for contributing your experiences.
If you can view the 720p on a PC, then a file with a 720p resolution is on Facebook. Maybe Facebook checks the bandwidth between the Facebook server and the Android phone and then downgrades the resolution automatically?
BTW, did you ever try the UVC FullHD Blender driver to try and get 1080p?
Also, to eliminate the possibility of incompatibility, can you try using one of the hardware or software solutions lists on the Facebook site. OBS is free and popular, so it might be a good tool to use for debugging the workflow.
Do you have 4Mbps upstream bandwidth?