It probably has to do with being native ipv6 and needing to ride a 6to4 nat to reach the broader internet.
Start at 1400 and walk the MTU down by ~50 until you find stability, then id creep it back up by 10 to find the 'perfect' size, but that part isn't really needed if you're impatient. :)
E. I found 1290 was needed for reliable VPN over an ATT nighthawk hotspot.