The increased heavy weapon ammo perk found on the raid boots and armamentarium cause you to lose heavy weapon ammo when you die or go to orbit. Every death or orbit results in about half your ammo being lost.
Without the perk you do not lose heavy ammo upon death or orbit.
I have found myself avoiding the raid boots because this perk is garbage for raiding.
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The issue is that when you respawn the perk bonus is not being applied until after the ammo is already given. This is a known issue that the team is aware of. Thank you for reporting it.