In Fusion 16, and the 2019.5 Mocha Pro plugin (on Linux), Mocha Pro seems to be doing an alpha multiplication on the insert channel when doing the insert. I’ve looked through both the plugin and in the Mocha UI, but didn’t find any place where I could turn this off. The plugin does have a “Multiply by Mask” setting, but that is already off (and doesn’t seem to have any effect on a rendered “Insert: Composite”.
I can make it work by putting an AlphaDivide node before the insert, but it was a little surprising that I couldn’t turn off the alpha multiplication within the Mocha Pro plugin itself. At least with Fusion, premultiplied images seems to be a bit more of the norm.