I just noticed that when I load Pro-Q 4, the Sidechain In port is not shown. I have to open plugin properties, disable audio buses and enable them back to have both Stereo In and Stereo Sidechain In ports shown.
It does not happens with some other plugins, such as Pro-C 3.
Quick update: I cannot reproduce it anymore. May be, Cantabile stores the last used plugin config. But I have never removed the sidechain port for Pro-Q 4 in the past, this is for sure.