How are you importing this custom geocoding? Would you be able to give us details on where you got this geocoding from?
If you're using this kind of technique: https://www.theinformationlab.co.uk/2015/06/01/uk-filled-map-geocoding-pack-for-tableau/
Then you'll need to copy the file into your tableau repository. Once copied, in tableau desktop you'll find the geo coding options when you try to assign a geographical role to a field.
We used a service (TAMU Geosciences), and now are getting them from a process with ArcGIS that our statistician uses - and just follow the process described by online support for loading custom geocodes. Tableau readily consumes ArcGIS shape files so we are exploring how to take advantage of that. We did not have any issues with past versions of Tableau. This firebird issue just popped up after I upgraded to 2018.3 the other day. I'm having it reinstalled to see if that clears it up.