Core 110 echo when connect hdmi cable to ingress?

Bit of a weird one,

We have a setup consisting of a Crestron MX70 setup using a Crestron Mercury as the desk microphone and speaker.

This is also a split combine room so as a secondary input there is a Q-SYS Core 110 running some ceiling mics and speakers . 

When the rooms are separate, running the Mecury's all is good. When the rooms are combined all works ok (manual change of microphone / speaker to use the ceiling setup) until you plug in a HDMI cable into the Mercury. The end users then get an echo.

Very weird..... has anyone had any similar?