I've moved this to the Mac category, and on macOS the $validate() scripting function is not available.
In your case, Mp3tag automatically replaces characters that can't be used in filenames on macOS (while preserving the contents of the title field, as already pointed out by @MotleyG).
If you want to use a different replacement character, you can use the $replace() scripting function, e.g., $replace(%title%,:,-,/,-) to replace the special chars by dashes.
There is also the configuration setting Replace invalid NTFS characters at Preferences → General → Filenames which is most likely already enabled in your configuration.
If enabled, characters that are not allowed in Windows NTFS filenames are automatically replaced by an underscore
_on field lookup. The following characters are replaced:\ : / * ? < > | ".
Enabling the automatic replacement also prevents folder creation when using the converter Tag - Filename with fields that contain the forward slash character
/.