I don't think any of the changes from 12.0.15 to 12.1 would have changed how that behaves.
I suspect you just hadn't run into a situation where /acmd was needed until now. Using /acmd is usually the same as using /cmd, except in a few edge cases. Generally, /acmd is the right one to use.
It works here, from a command prompt, whether or not Opus has focus. (Once I put quotes around the exe path.)
Have you got any empty folder tabs open? Or tabs showing something that doesn't have a real path? (e.g. The Computer folder.) The /cmd is probably being routed to a tab that cannot generate {sourcepath$}.