Ah, so you’ve edited the artifacts.gsav of that save, not a .json file, got it.
Yes, looks like the solution is updating the game.
Since you’ve already changed around some things, why don’t change some more…?
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First, backup your save, then create a new mod (assuming you want to update the game in the future and don’t want to make the same core file edit over and over). 
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In this mod, add in a new mutation: { "type": "mutation", "id": "ARTIFACT_CLAIRVOYANCE_EFFECT", "name": { "str": "Artifact clairvoyance" }, "points": 99, "valid": false, "description": "Adds the artifact clairvoyance effect back in.", "flags": [ "CLAIRVOYANCE" ], "debug": true }
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As for "flags", choose whatever your effect was:CLAIRVOYANCE,CLAIRVOYANCE_PLUSorSUPER_CLAIRVOYANCE. I assume it was the “normal”CLAIRVOYANCE.
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Add your mods id to the mods.jsonfile of your save.
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Open your game and load your world. This way you can also check if the newly made mod throws any errors. 
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Note the name of your artifact. 
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Exit the game. 
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Open the artifact_item_types.jsonfile indata/json/artifactand search for the name of your artifact.
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Note the id of the artifact with that name. 
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Open your characters .sav file. 
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Search for the id of the artifact. 
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That entry should have a "passive_effects"entry, which in turn should have at least either one of"has": "WIELD"or"has": "HELD"entries.
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In the “mutations” entry of that tag, add in the id of your mutation, including the double quotes (if you followed my suggestion, that would be "ARTIFACT_CLAIRVOYANCE_EFFECT").
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Save the file, load up the game and your world… Done. Aaand you should have your clairvoyance back.