It's to do with the Baron's side Goal.
Most event stuff continues to drop as long as any chapter is open. But a exception to this rule seems to be the Sampling/Tree drops.
They drop as long as the Baron's side Goal is open.
It's a goal you get when you need to upgrade your Hideaway. It consists of gathering the required items, and if you have them it auto-completes. If you manage to keep this goal from completing, you can continue to collect from the saplings/ trees.
To keep the goal open, you can deliberately NOT gather 1 of the required items for the upgrade. For example the Dust, which is easiest to get.
This has been discussed here previous. If you've gathered all event items you could in advance, you'll just have to wait for the game to ask for it again. Hope this explanation makes any sense