You don't need dtsdecoder.dll when bitstreaming DTS MA/TrueHD. Your problem is that mc doesn't bitstream.
If it would, your receiver should display DTS MA and not multichannel. MC tries to decode DTS MA. That's the reason why mc tells you to set up DSP settings. Therefore you need dtsdecoder.dll
There has been some minor changes with WASAPI output. Wasapi exclusive setting has changed for example.
You have to check your output settings again.
I have no problems with DTS MA-> Bitstreaming->WASAPI Exclusive Mode active.
My picture down below shows audio settings for one of my zones. Bitstreaming active.
Sorry for german language in the picture.
Output: WASAPI , but now you have to select the first option (picture) to activate exclusive mode.
(I know the picture shows spdif= no bitstreaming DTS MA. But i have the same settings with HDMI and bitstreaming DTS MA works just fine)
Check your output mode again.