devtrackers.gg
Sea of thieves went thru the same exact situation. All parties were open on launch and if someone gets disconnected midway thru playing, then a random would be able to take their spot almost right away, which in turn the party would have to back out completely and be remade. It took a couple months before invite only parties were a thing. Now the good news is that one of the devs mentioned in a video is that you do have that window of opportunity to join back in and maintain the loot you got throughout the dungeon, but wasn't direct on any sort of timeframe and also wasn't direct if that spot would be held for that specific person who got disconnected in the first place. Everything that I mentioned before is moot if invite only sessions are a thing, but I have yet to see any evidence.
There's a reservation system that holds your spot if you crash out. It holds it for a few minutes or so before it opens up for someone else (assuming it's public). 😊