What we have is some format strings using different rules from others.
e.g. in Convert Text file - Tag and Convert Filename - Tag single quote means single quote, whereas in Convert Tag - Filename and e.g. Action Format value single quote means "Output raw text".
This fits with the fact that the first type doesn't allow scripting and the second does. So I guess this is not a bug after all. But both are lablled the same on the UI - you have to look in the Help to discover there are two different types.
This is confusing and error-prone. I think the UI labels should distinguish the two types e.g. "Format string (not scriptable):" and "Format string (scriptable):".