Ben, My 4018C had same issue and now it is fixed.
It's caused by irregular Phantom power voltage of HM transmitter. It supplies 42V, not 48V.
And, MMP-C can not work with 42V.
I sent my MMP-C to DPA and they modified it to be able to work with HM transmitter.
Now my 4018C works perfectly with HM and HMa.