I agree that FormatJson should handle roLists the same as it does roArrays. However, I wouldn't call it a "buglet", since it states in the BrightScript Reference that "Data types supported are booleans, integer and floating point numbers, strings, roArray, and roAssociativeArray objects"; there's no mention of roList. However, the documentation is rather misleading as the first sentence states: "Formats an Associative Array as a JSON string". Yes, that's true, but it also formats arrays and primitive objects, not just associative arrays.
https://github.com/belltown/