First off, that shield is not a standard shield visually. You can clearly see that it looks more... ominous(?) than regular Arc shields. Of course, it IS a big Scorn boss unlike most shielded enemies, but still.
And before the patch, it WAS reacting to Stasis damage. The damage numbers did turn a Stasis-blue. There just wasn't any shield burst when broken.
Is there a miscommunication going on with the devs about this guy's shield?
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There might be and they might be waiting until a later expansion to fully release stasis shields after we continue to get more and more stasis weapons.