Note to myself: Do not use portal after boss fight! Please fix!

After I already lost boss fight loot (see https://www.pathofexile.com/forum/view-thread/3623077) I repeated the same mistake.

After the fight vs. Doryani I could not pick up the boss loot because my inventory was full. I decided to portal to town before talking to Alva/Doryani.
Big mistake! I was spawned at the beach of Act "4"! I rushed through the area to reach the first town, so I could get into the portal to get my loot from Act 3.
So I defeated the boss of the Beach and of course could not pick up the boss loot. I thought:

1. Get to town and empty my inventory.
2. Portal to Act 3 boss arena and pick up loot.
3. Return to Act 4 town and empty inventory.
4. Go back to the beach and pick up boss loot.

Terrible mistake. My act 3 portal did not exist. And I could not return to the beach, because there is no door back. So I lost the loot of two boss fights. Combined with the previous one that makes 3.

How is this even possible?!

I'm guessing (and this is only a guess) that you spent more that 8-15 minutes in town. Instances clear after that amount of time.

There does seem to be a related bug, though. I've seen both your portal disappearing when this happens (which seems slightly better), and what you saw - a portal that became unlinked.

A work around would be to drop less important stuff, grab the boss loot, then return to town. Hopefully your dropped stuff will still be there but at least if it isn't it doesn't hurt as much.

A message 'your portal is about to expire' would be nice.

The behaviour I see in PoE 2 is that portals to expired instances still exist but teleport you to the beginning of a new instance of that area.

The transition from Act 3 and Act 4 is the first instance that a portal disappeared for me.