You need to add two characters in this setting:
/
\
according to this message:
It’s already there:
Replace directory separators with
By default Picard will replace any path separators (slash or backslash) with an underscore when using a tag or variable value as part of the file path and name generated by the selected file naming script. This option allows the user to set a different replacement character other than the underscore.
This time I didn’t find a bug.