Hydro brake disk blackening
There's very likely something wrong that's causing a fluid leak. Hose connection, piston damage, or some other mechanical defect. They did the prudent thing before by seeing if it was a one-off/fluke situation, but now it's likely proven that it's not. What's really needed at this point is for a mechanic to inspect the hose connections in case that's where the problem is, and go to caliper replacement if not. This stuff happens not infrequently on new bikes these days, and in part depending on the brake model there can be some different reasons how it came to occur on both brakes at once. The shop I work at would cover this under warranty no question, and if it is as you say we would likely cut to the chase and just do two new calipers.