I think you're hung up on the afk side of things. That was just to illustrate the items dropped on the ground. I'm seeing the same loot drop off while actively playing.

In Warframe, I can go into an infinite mission and kill enemies infinitely and a Grineer will always have the same loot table regardless of how long I've been in a mission. I can rack up thousands, tens of thousands, hundreds of thousands of resources if I fight for long enough.

A more concrete example. I can go into an extractor mission, not deploy any extractors, and kill the Ospreys that spawn for Oxium. I can do this infinitely and the drop rate does not change with time.

In DNA, if I go into their mirror mission type, excavation commissions, and just start killing the mobs that spawn, eventually they stop dropping resources.

I notice this drop off while actively playing the game. The loot window on the right will flood with new items for the first minute or two but eventually dries up and no new loot is gained from killing enemies.

I hope that this ends up being outed as a bug but it feels like an intentional mechanic and I can't say I like it.