It's simple to reproduce. Create a quest, create an OSS, target the quest, place a hopper, use the hopper to deliver items. The items disappear, but the quest doesn't register.

Minecraft version? It might be relevant

I've tested this in my dev environment but cannot recreate your issue. How is your task and OSS setup?

Alright, I gave it a try with forge 1987 and BQ 130. It still happens. You can try it with the 16x Stone -> 1n quest in this collection.

Try setting the task to ignore NBT

That doesn't change anything, it still happens. I'll see if I can try it without any other mods. Maybe it's a bad interaction.

It seems I was testing on the wrong version, my bad. Issue now confirmed. I'll try and get this fixed up for next update.

EDIT: I've found the bug and fixed it

Sorry for the late response. This was tested with forge 1969 and BQ 126. I'll confirm that it's still a problem, and if so, get back to you.