I have an ASUS laptop and a Sennheiser headset. When plugged in the mic isn't working.

[hesof4]

I have a pink/green splitter, doesn't make a difference. Laptop mic gets disabled, headset mic is not recognized. UGH!!!
Additional info: Windows 10.

 

[Unkk]

There are two different specs for single plug audio cables.
Here's a diagram of what I mean:

It's likely that the mic and ground are revered on yours.
This can cause issues when the mic signal is used for common on the headphones.
I've had this happen too.