I was stumped at to why my translation for Gravity Forms was not working.
And then I looked in the languages directory:
Hmmm... that doesn't work when Loco Translate outputs
Sure enough, adding the prefix made the translations show up.
I looked at a few more plugins and Advanced Custom Fields does this too:
Now I think GF is the odd one here, but it appears that being able to specify a prefix (or to indicate the textdomain should be prefixed to the .mo file) would be a useful option.
Or perhaps outputting both (just to be safe)?