Sorting by field from custom database table

Sorry if this is a duplicate. It looks like my previous post for this did not work for some reason.

The sorting options do not seem to work when the meta data is coming from a custom table. Much like this thread.

I would like to sort the posts by the field ‘postcode’. However it does not list alphabetically – https://i.imgur.com/XjoAeFP.png

My code for registering the CPT and meta fields

Both extensions are enabled https://i.imgur.com/DJEwxxF.png

Using AIO v1.10.10 and MB v5.2.4

Am I doing something incorrectly?

Thanks!