Mono MixConvert 6 plugin on Phone channel in CR?

i cant get the MixConvert plugin to make mono mix on the Phone section on the control room.
i insert the plugin on the insert slot on the phone section, and when i switch mixconvert to mono, it cuts the right side signal and only the left channel is heard on left side.
it literally cuts the right signal on the insert slot routing.(checked it with nuendo)
not a big deal actually cuz i can use the “Stereoenhancer” plugin or any other plugin that can sum to mono but i wonder why that’s happens