I assume you've already done this - but in the Hue app have you gone into the HomeKit & Siri menu and made sure the accessory is listed under Accessories there?
Edit: I say this because I frequently find that HomeKit and Hue bridge get "out of sync" so I have to go into the app and go to that section and re-sync them. I have no idea why they can't figure that out on their own, and it doesn't prompt me or anything, I just discover it sometimes when I bother to open the Hue app.
Funny, your post made me realize there is no HomeKit forum here.. Maybe the macOS forum would get more replies for this question though since HomeKit is part of the macOS broadly, or maybe one of the iOS forums, or even Apple TV or HomePod since they are hubs by default.
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