Hopefully I describe the issue alright. I have a report from players that the backback "gather" has an issue. If they have say for example scrap in their backback, and gather is toggled to existing, or all ... it will not automatically put any NEW scrap they pickup into the backback unless a single spot is free in their main inventory. So as long as they have a spot free they can run around and collect, the minute a spot is not free in main inventory, backback basically stops accepting items as well, even though it should still be adding resources, or scrap to the backpack to meet stack size etc.

This a known issue?

This is a known limitation, due to the way gather mode is implemented, which is that it watches for new item stacks being successfully added to the player inventory, by attaching a callback function directly to the player inventory object. I'm not currently planning to redesign how gather mode works at this time because alternative designs (such as watching every item move attempt in the server) would be more complex and/or more performance intensive. This is also a limitation with the Bag of Holding plugin's gather mode feature as it uses a similar design.

Weird, I don't have this issue as an Admin ... I have filled my bag, plus my inventory before doing cleanup and never run into this. lol 
Appreciate the quick response man, your always so fast jumping on help, updates etc. great to see!