Trying to check for dropped items with a custom NBT tag

I'm having a problem detecting items dropped by players upon death. I've got it figured to where when a player dies, the items that drop are assigned a custom NBT tag. However, I'm trying to run a function that detects when the tagged items are about to despawn, it doesn't seem to register or check for the custom tag? I can detect when an item is about to despawn, just not when the item has the custom tag and is also about to despawn