Needs repair possibly the ceramic radiants are cracked and allow the flame to light at the injector I would advise you don’t use and either seek repair or get a new one as it’s dangerous
BES.co.uk | Next Day Delivery On 15,000 + Plumbing Supplies - https://www.bes.co.uk/40-mm-1-1-2in-water-waste-big-boss-pipe-adaptor-black-19871/ This maybe