Sorry.
But I think I figured out what was happening. When I rename the module the same name as the function itself, I would get "Undefined function 'concatrelated' in expression."
When I named it anything else, it works fine. I didn't even think to rename it something other than Module1 or ConcatRelated. Must be similar to naming a text box the same name as the field it's referencing.
Sorry, I feel pretty foolish.