Changing the contents of the offhand slot via the /item replace/modify command creates a vibration which triggers sculk sensors. However, doing so does not actually produce a sound event (presumably this is meant to be minecraft:item.armor.equip_generic). Likely related to MC-249834.
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Duplicate of MC-249834
That does seem to cover the issue as far as item swapping goes; not so much the /item command.
(Go figure I spend 20 minutes searching for variations of "offhand" and "sculk sensor" only to miss the duplicate report because they hyphenated "offhand.")
I can confirm this behavior.