You are simply not remembering correctly. There has never been an indication when Zen moment is active. I even recall a few posts asking for this as a QOL change.
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For some reason I distinctly remember a buff icon while it was active. But videos from year one agree with you. However, that still leaves the question of whether or not it is working, because it should be 100% kicking in the moment I hit, since I KNOW that's exactly how it worked seasons 1-3, as in the case of Hollow Earth. Of the 5 guns tested, none of them showed a sudden enormous increase in stability.