This is normal.
You can not route a stereo channel into a mono Child Buss.
The Child busses are “hard wired”, so the in- & out- should be equal.
In a “normal” bus routing, when the channels don’t match, MixConvert is automatically inserted and a downmix is performed according the preset system. When a channel is hard-wired, MixConvert can not be inserted into the chain.