I'm on the 11 Pro Max as well and these same reboot issues happen for me mainly at the lock screen. May be something with FaceID causing a kernel panic which reboots the phone. I recorded no reboots overnight when I am not using the phone, but they increase to 8-15 per day depending upon how much I use the phone. I went back to 13.6.1 over the weekend, and had no reboots, however my watch is on 7, so I'm trying to stick it out on IOS 14 and hope for a fix. Feedback has been submittedBeta 5 has been nothing but unstable on my 11 Pro Max, which is a shame as Beta 4 was rock stable with no issues.
Pretty much guaranteed to restart any time I lock the screen, has restarted 2 or 3 times when first opening an app after unlocking, and apps occasionally crash to home screen.
Have reported it via feedback app.
Hopefully Beta 6 comes out this week and fixes these things.