The game never tells me I have a full postmaster in an activity or patrolling an area.
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I highly recommend having Destiny Item Manager open on your phone or an extra monitor to help keep an eye on your postmaster. It's not the ideal fix, but I haven't lost anything in a long time to the wave of blues. The game won't update your postmaster with all the engrams and loot lying around until you go to orbit so defs try to pick up the stuff on the ground and watch your DIM when you're staying on a destination for a while.