So I do know what’s causing it but this happened twice now.
The first time I was playing as the behemoth on I believe the map is called weather control. Was playing everything was working just fine then I got to stage 3 everything still working fine it was only once combat made it right next to the relay that the whole game froze up. I thought maybe I lost connectiong but nope my connectiong was fine. I waited to see if it would fix itself but it didn’t so I just left
2nd time same thing except this time I was not the behemoth someone else was. Everything works fine he makes it to stage 3 fight around the coast line everything is going fine but as soon as combat got around the relay the whole game froze again.
I don’t know if it’s specific to the stage 3 behemoth but I know both times it involved stage 3 behemoth would have to do some testing to see if it affects all stage 3 monsters.