Pretty sure it's related to the old (and recently patched) ammo bug. Basically, spawning would spawn ammo in your weapon first, spawn your armor and then add stats for carrying more heavy ammo. This resulted in a loss of ammo. For instance, you have a Gjallarhorn loaded with 7 rockets and armor that let's you carry more heavy (which is why you have 7 rockets not 5). You spawn, weapon first, armor second. Your Ghorn can now carry 5 rockets, the game knew you had full heavy, so it maxes you out at 5 rockets. Now your armor spawns a fraction of a second later and you can carry 7. But you lost 2 rockets in the process. This used to happen EVERY time you died. It used to make wiping in raids insanely stressful and was the cause of people buying stacks and stacks of heavy ammo synthesis. Bungie patched the ammo loss on death bug, which they claimed was extremely difficult. My guess: they didn't want to spend the time on the whole changing location ammo loss, since it would be just as time intensive for an issue that's less of an impact on the game than dying and losing ammo.

The game attempts to restore the same fraction of heavy that you had, but its bugged and drops your heavy

This is deliberate to prevent players from starting new activities with maximum ammo for heavy and special. It's been like this since the game launched.

Did you change armor to you can only hold 5 rockets? Like switched to non extra heavy armor?