Nothing useful to add except to tell/ask the BOCHK staff when you hand back the form that they’re sure this is how BOCHK does the withholding for their other current customers that get <30% deducted.
No exp in HK/BOCHK/Eu - my exp is they follow the form. If you fail to give them/refresh the W form when they ask for it or fail to include the correct details (the section on the tax treaty your status belongs to), they will assume there is no beneficial tax treatment and deduct the full amount % for that period until you send in a correct form. Even if the last deduction is a diff % and all other records indicate you can get beneficial tax treatment.
Good luck 😬