As far as I recall, (don't have access to wow or Archy atm) there's a quirk with RequestArtifactCompletionHistory() where the results returned from the server are invalidated by crossing any portal in the game (even a zone to zone portal) and you have to re-request the info.
Relevant changelog entry from Archy
- Fix so overal progression and solved counts remain consistent.
Crossing any portal in the game (instance or otherwise) invalidates the client Artifact History who woulda known...
Full changelog of versions 1.8.10 to 1.8.24 when I was doing most the bugfixing.