According to RFC 6350:

6.7.1. CATEGORIES
   Purpose:  To specify application category information about the
      vCard, also known as "tags".
   Value type:  One or more text values separated by a COMMA character
      (U+002C).
   Cardinality:  *
   ABNF:
     CATEGORIES-param = "VALUE=text" / pid-param / pref-param
                      / type-param / altid-param / any-param
     CATEGORIES-value = text-list
   Example:
           CATEGORIES:TRAVEL AGENT
           CATEGORIES:INTERNET,IETF,INDUSTRY,INFORMATION TECHNOLOGY

On reading a VCARD, a tag entry should be added to the database. On saving a VCARD, the internal tag should be substituted with its value.

The tag is an implementation detail, and only valid for the local akonadi database.

Same happens when importing/exporting vcards, "Categories: Foo" is ignored on import, export uses "akonadi:?tag=.."

Reproducible: Always