There are two ways to enable/disable the automatic adding to lists. The first is in Preferences-Lists. The second is via the function button to the left of the field via the Enable/Disable Auto-Saves menu item.

I've written a simple action to add the Composer field items of all selected files to the Composer list. It can be run stand-alone, from the Multi Field Editor or from the function button to the left of the Composer field:

https://2manyrobots.com/actions/Add-Composers-to-Preference-List.zip

BTW: Add to List didn't work as you had more than one file selected ... with different Composer fields.

Thanks! I'll give it a go.

It's very easy, when one is tagging as many classical albums & box sets as I seem to be, to forget to manually add each new composer as they occur. I can't remember why I switched off the auto add function. I think maybe I hadn't settled on some spellings (Rachmaninov/Rachmaninoff, the million ways to render Chopin's first name, etc.) & I was getting duplicates with different spellings.

Anyway, hopefully this will solve the problem.

You have Title selected in the MFE. The action only supports the Composer field.

You can also directly run it from the Composer field's function button's menu:

Multi Field Editor Actions>Add Composers to Preference List

That works a treat, thanks!