Smell after using instant water heater

Hi, I have recently renovated a 20-year-old resale flat. I realized that after a shower with hot water and sometimes during the shower, a weird smell will be emitted from the water heater. I use a mistral instant water heater MSH103. Why I think it is from the water heater because we have opened up the cover once while using hot water, and the smell seems to be emitting from there. It is a weird smell and it is not a sewage smell. After a shower, we turn on a ventilation fan and close the door, usually after 10 minutes we open the door, and the smell fills the whole toilet. Sometimes the smell also concentrates near the toilet bowl there. It is quite weird. It only happens when we are using hot water to shower. The smell does go away shortly tho. We have gotten the vendor to replace the water heater once but the problem persists.

In my opinion, there are no chokes in the pipe because there is no flooding/ponding. It is also not a sewage smell; it is only present when we shower with hot water. Any ideas gurus?