I downloaded your inventory to try to figure out how it works and how to add that container functionality and while I was messing around with it I noticed that if you are overlapping the pick up volume of multiple objects the last object you overlapped with is the only one you can pickup until you leave the volume and come back in. I think you could avoid this issue by using a linetrace and picking up whatever actor the ray hit instead of using a volume.
If you end up trying to add container functionality please post an update. I’m trying to add it but I’m not very good with blueprints so I probably wont be successful.