When data is imported from a spreadsheet – if the custom fields do not already exist inside your account then they are created during the import. 

If the custom field on the spreadsheet does not contain a tag we ‘presume’ the custom field should be created with no associated tags and therefore applied to all contacts.

However if you specify a tag (tagname:customfield) then we associate the custom field with the tag.

If you then import new data into your account, if your existing custom fields do not have a specified tag associated with them on the spreadsheet it is again presumed that the custom fields are to be added against all contacts. 

On your data import spreadsheet you need to put in your custom fields but have the tag group specified for the custom field to be associated with that tag group and not all contacts as shown below in red:

The tag is 'in-house-training' and this is associated with the custom field 'Budget to spend £' 

The other example shown is the tag 'new-lead' which is associated to the custom field 'Courses interested in?' 

By importing your data with the tag custom field association in place this will then correctly associate those custom fields with that tag group - as shown below inside a contact record: