stuck in "Blooming Field" with no Boss

Boss Arena is accessible first, but after slaying the mob inside there's no way back (missing "portal") and door is closed.

Exact same scenario here. There is a rare mob inside the Crowbell boss arena, the door closes behind you, as if it triggered the script for fighting the Crowbell. You kill the rare, the door doesn't reopen, and as far as I can tell there isn't any simple way to get out.

I've had someone join the party and portal in, but of course they just ended up in the same spot as me.

Bug Report ID: #3403344012
Character name: FemboyFreebooter

Steam screenshot of me trapped with the door closed: https://steamcommunity.com/sharedfiles/filedetails/?id=3561580596

This is a "hindsight is 20/20" thing, but the best solution for now might be to NOT make a portal in the boss arena, and just exit out of the game, rejoin, and hope it plants you at your original map spawn.