As Hellfire pointed out the Aux is input only...maybe it's your explanation of the issue.
The Aux Input for the Surge Module (
Medeli DD512) is adjusted from the source (phone,tablet etc)..not the module (Volume adjustment Knob for phone/line output only...menu volumes for kit/voice..[<] + [>] in Song Menu).
Aux input is not recorded, if recording on the module.
Are you losing the Aux input in the Drum Mix?...If it just started occurring and you are using the same Volume output settings at the Source you have used prior,I would check the 1/8" TRS cable..they go bad.A small blast from a canned "Air" duster to blow out any dust at the port sometimes solves it.
A Factory Reset may help also (Hold down [<] + [>] simultaneously while powering on)..but you will lose your trigger settings etc , so make a note of them.
The Surge Module (Medeli DD512) with Harness is in the $75-150 range depending if new/used..with or without the trigger harness,which is also the price range a repair shop with a Minimum charge might charge you just to diagnose it.