There are three different types of changes:
1. Reference: saved to DialogTaggedItem, and only contains a pointer to the tagged object.
2. Snapshot: saved to QBMTaggedChange, where the column ChangeContent contains the object XML.
3. Change: saved to QBMTaggedChange as well, but the ChangeContent contains the differences only in an XML format.
Up to and including version 7.1.x, SchemaExtension saved schema extensions as DialogTaggedItems. Consequently, when using pre-existing tags, only those that already contain DialogTaggedItems are usable.
From version 8.0 onwards, SchemaExtension saves schema extensions as QBMTaggedChanges. For pre-existing tags, only those containing changes are usable for schema extension. This behaviour is intended.
However, versions 8.0.1 and 8.0.2 do have a product defect designated #29153 where the SchemaExtension frontend filters the tags incorrectly.