I have the mod unlocked on my seasonal artifact (flame fiber freeze) but i only see Thermodynamic Siphon(Solar/Stasis) in the mod list for the helmet. I've checked with a clan member and he's not having this issue.
https://steamcommunity.com/sharedfiles/filedetails/?id=3220606281
https://steamcommunity.com/sharedfiles/filedetails/?id=3220606227
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Within your artifact can you un-equip it then re-equip it?