the arch illager is able to teleport outside of the arena onto the platform where the jump pads to the arena are

1. go to obsidian pinnacle
2. reach the boss arena
3. roll off the arena
4. the arch illager may teleport to the location of your respawn point

note: if he has summoned a redstone golem, then defeating him while doing this glitch (provided that the golem is still alive) will prevent the heart of ender from spawning until you defeat the golem. also any pillagers or vindicators still alive at this point will not be instantly killed. i'm not sure if those are required for the heart of ender to spawn too. please let me know if you manage to reproduce and find the answer to this question.