Tag panel doesn't update - bit overwhelmed!

I am brand new to mp3tag and trying to muddle my way through a task.

I have DL a full discography of a record label from bandcamp.

I am trying to get the catalogue number for each release and place it in square brackets before the album title.

Bandcamp does not include catalogue numbers as part of the delivered metadata so i have loaded my 731 tracks across approx 110 albums into mp3tag and manually pulled the metadata and extended data from discogs album by album uisng the Tag Source function.

Discogs provides me with 'CATALOGID' as part of the extended tags - exactly what i want. For a subject album the album title would be 'Split' and the catalogue id would be 'PURR 0033'

I want to take this data and place it lower case in square brackets before my album title. To do this i have created a new column called 'CatalogID' with the value as '$lower(%catalogid%)'. This correctly gives me my column with the lower case catalogue ID for each album. For my example this would be 'purr 0033'

I've then customized the 'Album' column such that the field reads ''[$lower(%catalogid%)]' %album%' For my example this gives me the result i desire in the 'Album' column '[purr 0090] Split'. Checking all other albums I am getting the result i am expecting.

I'm now patting myself on the back thinking my task is complete, however, the Tag Panel on the left hand side is not reflecting my Album column, with the Album data still referring to just the album name, no catalogid. I have saved all the tags with no change.

I am sure there is something very obvious i am missing but somewhat overwhelmed with the terminology in the documentation. Is there a step i'm missing or i am implementing the change i want in the wrong manner? Any help appreciated. Screenshot provided for context

So far you have only modified the display in a column.
You would have to really write the data to the field ALBUM with an action of the type "Format value" for ALBUM
Format String: '[$lower(%catalogid%)]' %album%
(afterwards you may want to revert to the pure display of a single field in the column as you would otherwise see the catalogid twice.)