Follow-up: there’s an (erroneous) line in the ref script that invokes a different section of Lua code, which itself returns a value of 1, which appears to be the rogue value that makes its way out into the game. This has gone from a case of “I don’t know why this single case isn’t working” to “I don’t know why the other instances aren’t breaking!” If anyone finds this thread in a Google search or something: don’t invoke Lua from inside a ref script.
So much for keeping parts of my code separate!