I've unlocked 25 of my season artifacts
However, only 24 were unlocked in the season achievements and were not completed
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Hello, Bungie is aware of this issue, and are currently investigating. [quote]• The Synaptic Fanatic Triumph may not unlock for some characters after unlocking 25 Artifact mods.[/quote] Via the [url=https://www.bungie.net/en/Forums/Post/260598792?sort=0&page=0]Known Issues[/url] forum page.