I actually just had this same issue about a week ago. The code to handle pecking depth reduction is absent in the UMC-750 postprocessor. I changed it using the code from the Haas VMC postprocessor which does have it.
You will want to change the deep drilling case to the following red colored code. It should be under the onCyclePoint() function in the UMC-750.cps:
case "deep-drilling":
writeBlock(
gRetractModal.format(98), gAbsIncModal.format(90), gCycleModal.format(83),
getCommonCycle(x, y, z, cycle.retract, cycle.clearance),
// UMC-750 ORIGINALS
// "Q" + xyzFormat.format(cycle.incrementalDepth),
// "P" + milliFormat.format(P), // not optional
(((cycle.incrementalDepthReduction > 0) ? "I" : "Q") + xyzFormat.format(cycle.incrementalDepth)),
conditional(cycle.incrementalDepthReduction > 0, "J" + xyzFormat.format(cycle.incrementalDepthReduction)),
conditional(cycle.incrementalDepthReduction > 0, "K" + xyzFormat.format(cycle.minimumIncrementalDepth)),
conditional(P > 0, "P" + milliFormat.format(P)), // optional
feedOutput.format(F)
);
break;