It's a mixture of both wierd loot tables between chests and horrible RNG.

The left chest on turn 1 drops a total of 4 items, two of which are maiming (earrings and belt as well as aiming bracelets and casting ring), that's a good part of where the drg loot shower comes from. Everything else comes from the right chest so you've got a dramatically lower chance of getting what you need from there. The other chests are not quite so well documented but I suspect the issue isn't isolated to turn 1.

Otherwise yeah, RNG sucking plays a big part in it, I've seen 1 healing drop over the last month of 1-4 clears. Frustrating isn't the word.