Several seasonal pass bonuses grant extra loot at the end of an activity, like strike armor, gambit armor, Temporal Clause, Pluperfect etc.
The problem here is that they don't drop from engrams so they DO NOT GO TO YOUR MAILBOX.
I had the strike chest pop open and fill my kinetic slot with blues then no room for Pluperfect. Figured no big deal, like all full inventories the loot will go to the mail - but it did not. There was no official drop in the world for this to prompt to go to the mail since they go directly into your inventory
The same will happen if your armor slots are full for the seasonal bonus armor, or heavy is full and a Temporal Clause which is mega rare already to drop procs then you fail to receive it, they will be permanently lost. I usually keep 2-3 slots open for drops but you can't really control what random loot you get at the end of a strike/gambit/crucible game goes into your inventory then for the seasonal bonus to proc. These not dropping from engrams is the same reason why it goes into your inventory at a base 750 level rather than your current level less 0-3 power
I bet you the LMG and Auto are less "rare" than people think because they've had full inventories and havent noticed it drop at the end of a game to get auto deleted
EDIT - WARNING: this also happens with Divinity. Forgot I was almost screwed out of this when my energy slot was full, looted the Divinity chest and said no room with an X over the icon. Just to make sure I scrapped an item then was able to re-loot the chest to get Divinity, otherwise I would have lost it from not going to the mailbox
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