Bungie is aware of this.
https://www.bungie.net/en/Explore/Detail/News/51177
[quote]The Synaptic Fanatic Triumph may not unlock for some characters after unlocking 25 Artifact mods. Players can log into alternate characters to complete this Triumph if the characters haven’t unlocked all the mods yet. We’re investigating a fix if all characters have unlocked the mods and haven’t received the Triumph yet. [/quote]