I'm pretty sure this was an intentional decision by Bungie, so don't expect a fix. The special synth ammo drop both increases the pay off of farming, while simultaneously adding a new cost. This, I think, serves to address both problems with the loot cave (according to Bungie): the activity wasn't actually any better than just playing the game in terms of payoff, and it was not 'playing' the game that Bungie envisioned. The increase in payoff: the ammo synths, rare engrams(glimmer), upgrade materials, glimmer, DLC currency, and rare Key and Legendary drops now makes farming an actually viable way to get diverse in game resources relatively quickly, and more importantly, consistently on an apparent and predictable timer. The inclusion of specific content associated with these new chests, from the Faction Bounties to the cool new scorch cannons and what-not make it clear that this is a part of the game that we are intended to play with and experiment. In order to balance this easily abusable stream of gear, as well as keep players actually playing the main parts of the game, Bungie has ensured that any players who are overutilizing this route to loot find that their ability to actually collect and store loot is diminished temporarily. In other words you having increasing cost that scales with use, which should result in self regulation of use to the amount that is optimum for their own in-game needs. For example I don't currently have a key for the PoE, and since the weekly reset, I'm ready to try for another exotic. So I'll be willing to keep hitting those chests until I hit a key, and I can expect to be playing special weapons to thin out all those synths from my inventory. Without the strong pull for a Key, I'd probably only hit those chests in order to grab synths when I'm running low, or if I need a low intensity way to grab some glimmer in between work or chores.