How are drops treated on (fast) travel to a different area?

When I drop a purple weapon near the New-U in Serenity’s Waste, sometimes it stays around when I leave for the The Nexus and return, and sometimes it doesn’t.

Why? Dropped it too close to the New-U?

I don’t think anything less than about Blue tier rarity sticks around ever? So it’s also a function of rarity?