After patch 1.1.1 I noticed that heavy ammo count stayed the same after death/respawn but changed between missions. To be clear heave ammo goes missing after returning to orbit and starting another mission. Please see my videos on Youtube for examples of this.
http://youtu.be/WfV6ZYif4Js
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Bungie has stated that this is intended. They were thinking about changing it early on this month but ended up keeping it the same.