2018 Intel Mac Mini, Mac OS 15.7.4 (24G517), MP3Tag Version 1.10.3 (133)
Trying to use search/replace action for filenames doesn’t actually work. Using a different field like Title functions fine.
The correct field name for this would be _FILENAME (note the preceding underline character. The same is required for _DIRECTORY if you wish to use the Format Tag Field Quick Action for that as well.
It may be easier to use the Converter>Tag - Filename instead.
Why is there a FILENAME in the list then? How is a user supposed to know this? It would seem like a good thing to correct/better clarify in that list.
There are things Search/Replace can do faster vs Converter>Tag - Filename and also things it wouldn’t work for using the later for. It would be nice if Actions could be given an option for buttons or keyboard shortcuts.
If you manually type this in once it becomes part of the list. If you check the metadata of a file you tried to update this way you will find a new custom tag there with this name.