Ok,
so in my testing I can say there is some form of noise reduction on the Mic 2 transmitter ALL the time  (with OA4) and when you turn on "noise reduction" it just more aggressively applies it.
If noise reduction is off it should be off completly.   When I plug in a 3.5mm Lav mic like the etm-001 with an extension cable the sound is miles better.

Thank you for the feedback. This has been already forwarded to the relevant team. I will keep you posted for any updates. We appreciate your patience.