Maybe this is a defined behavior but it looks like an error.

1. Simple autofarm with oak trees
2. Farm collect logs, apples and saplings
   The problem in a saplings collect logic - they are all going to output instead of filling internal buffer first. So it leads to internal saplings buffer emptying and farm stops working

Forgot to add details:
MK: 1.12.2
Forestry: 5.8.2.409

This is defined behaviour so you can automatically fill a new type of saplings in without worrying about the old sapling type.