For far too many YEARS now, people have been able to don sentry gear in Cyrodiil (this is a PVP set), activate the ability to be able to see people in stealth -- with no range limiter which is insane btw and an entirely different topic of conversation -- then rapidly change back into their main 5-piece sets via an add-on like Wizard's Wardrobe, which makes gear swapping near instantaneous, and then go about their merry PVP way to assault their target.
This is a bug in the set and an exploit by users who perform it. NO SET should retain any buff, even if a duration buff, once that 5 piece bonus condition is no longer met by virtue of no longer wearing said 5-piece. I'm not talking about back barring a 5-piece set, that's different as YOU'RE STILL WEARING THE GEAR. I'm talking about actually removing the items to put in place other items so you have more damage, tankiness, etc.
Fix Sentry. Either add the condition that once no longer worn the set buff drops, or change it back to the way it used to be by giving the wearer a range bonus to seeing stealth/invis and again make sure it drops if the gear is swapped.
Sentry swapping is the definition of cheating: to give unfair advantage to a player over another through duplicitous means -- and if it's present within the game it's an exploit as there is no way this is intended as you ZOS are then giving players access to 3 5-piece bonuses.
Either way, fix it.