Are you using a weapon that already has either an intrinsic champion perk or one equipped on the artifact?
Radiant won't apply anti-barrier if the weapon already has another anti-champ effect from another source.
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Ahhh! That could be it. Tyranny of Heaven has unstoppable. Would Riptide also be negated then since technically it can stop them even though no mod??? Thanks!
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Riptide is not on the artefact so it would be fine
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Oddly the riptide will work as anti-barrier while you have radiant but it then won't stun overloads on slow or unstops on freeze unless you don't have radiant.