Bungie is working on this. There is a heavy ammo penalty on death no matter what, but the loss from wearing gear that boosts your carryable ammo is unintentional. They have said that upon respawn the game is registering your respawn before it registers your change in ammo from armor (it supposed to work the other way around), causing you to loose more ammo than you should be. They are working on a fix. But it was confirmed that there is an intentional heavy ammo penalty on death, so no matter what you will lose some even if the fix it, just not as much or as quickly.

I have this problem to a greater extreme, as do my brother and a few friends. However, more than one freind has never had this happen, so its not a game wide glitch and stacks the odds in the favor of those individuals Say I'm using a rocket launcher, when I die, I will always lose two missles. It doesnt stop at 5 if was 7, it will go 7-5-3-1 (it wont take away the last missille). Synthesis are more expensive for me then others do to this, as they may not payout , as I can lose almost all missiles before I fire even one. There is little point to me using armor that boosts my ammo cap do to this

I believe this is a known bug that they are working on.

If you die then your ammo will always go down. Or it should at least.

I have the same issue with both Armamentarium on my Titan and my Raid armor on my Warlock, both which grant the extra heavy ammo perk. Anytime I go to orbit or anytime I die, I lose heavy ammo.