https://www.gamespot.com/articles/destiny-2s-mida-multi-tool-exotic-quest-has-a-seri/1100-6453210/

Update: Bungie has noted that Destiny 2's inventory issue may impact not just the MIDA Mini-Tool being lost, but also the Man O' War. Quest rewards that you don't have room to acquire should normally be sent to the Postmaster, only that isn't happening properly for at least those two items. As such, you should be sure you have multiple free weapon slots available when completing these quests. The original story follows.

Destiny 2 developer Bungie has confirmed it's aware of another issue with the game and is currently on the case to fix it. The official Bungie Help Twitter account today acknowledged a problem where the MIDA Mini-Tool does not get sent to the Postmaster after completing the "Enhance" quest.