Hmm. Potentially a tiny bit weird, but!
I just caught a bug in the bridge install (not the hotfix) that was preventing it to update the dispatch script properly.
And, interestingly enough, older dispatch scripts can't deal with the current MRPI version properly.
So, maybe not so mysterious after all

.
EDIT: Yeah, after double-checking some stuff, if you'd never installed a hotfix and/or depending on the age of the hotfix, you'd potentially be either missing the dispatch script, or be stuck with an old one that doesn't actually launch current MRPI versions.