4k video randomly stops recording

I have an iPhone 11 Pro Max 256gb recently updated with plenary of available storage. I regularly use it to record stand up sets. Recently my phone has stopped recording in the middle of my sets. This has happened twice now. I’ve restarted my phone, made sure that the battery was fully charged, turned off the Wi-Fi, force closed any active apps, confirmed that there were no available updates and still it’s happened again. My phone was on a tripod under and AC vent in a well ventilated area. What am I doing wrong? Why is this happening :(