I am having the same issue. Screen shots attached. Guardians are swimming around on the conduit I have the buffs. No attacks. Conduit built completely (as per the wiki. Conduit is active:
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Guardian doesn't care.
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Final SS taken in Creative mode to show complete conduit structure.
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I disassembled the entire structure and rebuilt it (in creative mode) and it started working. Not sure why. As best as I can tell it was block for block identical.
Oh yeah, Version 1.16.5. Should have put that in earlier.