I was in the top 20 on Durandal as well, and our LS was regularly pulling off 40k scores out of a theoretical 45k cap at Hamlet Level 2.
Out of 19 of these wins, the chest dropped nothing but dark matter and the occasional ixali currency. For every LS run. the type of chest was random, we saw bronze silver and gold on occasion, but the consistency of the rewards from chests was blatantly lopsided, in that Chest rewards netted at the top available tier was dark matter and a small amout of presently worthless beastmen crrency... but completing Hamlet Defense with under 15k points score rather consistently rewards a "consolation prize": Magicked Prism (Indigo Star).

My wager is that they have a chest loot list in mind for people in the following tiers:

Tier 0 Under 15k-----chance at Indigo star
Tier 1 15k-30k-------?
Tier 2 30k-45k-------?
Tier 3 45k-60k etc etc

The problem is, they may not have anticipated that before May 10th (when Aleport and Gold Bazaar go live), that people would be regularly exceeding tier 1 and tier 2, so the loot list for the chest was coded to give generic Ixali currency and dark matter, and the worthwhile gear would be added in 1.22a .

If I am wrong on this, I will likely not be participating as often, nor I wager, will a large majority of the server populations.

TL;DR -- If the only way to get anything but dark matter from the chests is to perform poorly, where is the motivation for groups to keep going at it, especially if they aren't in the top 20?

Goddess, I hope they fix this soon.