How work instances?

I’m experiencing a lot of strange things with dropped items on the floor.

I completed a ND but had full inventory so that I couldn’t pick up a precious heart just dropped, so I tele to tree of whisper and in front of the vendor here I also opened a whisper chest. There was a ND key I couldn’t pick up, so I moved a potion to stash and decided to be back to ND to pick up my heart, much more valuable that the key. Being back to my vendor, my ND key was disappeared. It was all very quick, just the time to tele, pick up the heart and tele back again.
Ok, no problem, not very important… I stayed in the game for I think further 30 minutes, every time being back to the same location (stash in tree of whisper location)… After 30imutes the ND key appeared again on the floor…

More: after 3x50 blood event in BH, full inventory, then come back to vendor to sell stuffs, and when come back via portal half of items are disappeared and half still there. How is possible?

Same thing with legendary on the floor… Some, eg 5, are moved to temporary “lost tab” in the stash (which have 10 slots) and some remain on the floor. Which logic is this?

These strange things with items on floors are not new, and I think something strange happens with instances when we teleport to town… But the ND key re-appeared after 30 minutes on the floor is very strange, as I came back to a previous instance in which that item existed.

The instance can only handle 12 players. When you left, another player filled your slot and you teleported to a different instance. Happens in blood harvest and at world boss all the time.

Pretty much any area that has a lot of players. Hell Tide would be another one.

Lol… I know how instances works… but Blizzard should avoid that mess I described. I’m not leaving the game, so why force me to change instance. I have my 1/12 place in instance X, let me stay there until I leave the game

Always online live service. Unfortunate but true, hope to see something situate this in the future also!