Why am I invincible while playing Super Mario Bros Deluxe?

So I have been playing Super Mario Bros Deluxe on an emulator (don't judge, I legally own the game but it shorted out after I dropped it into a sink full of water like 10 years ago, before I got to even play it) and I'm not using any cheats but as soon as I got a fire flower power up I haven't been hurt whenever I have hit into anything, only dying when felling off the map or not making it in time. Is there any explanation for this?