Check for reversal of L-in and L-out wires.
Unless you built a scene in the app that is looping I would say you have a wiring problem. The wire for L-in should be always hot with respect to the neutral no matter what state the switches are in. If you are safe and comfortable with a volt meter you can check that without killing yourself. If not, hire a professional.
WARNING! Mains voltage is LETHAL. If you don't know what you are doing, and from your question I suspect you may not, don't even attempt it. Hire a professional. This measurement is taken while the circuit is live so wear insulated electrician's gloves and observe all safety precautions. The first time you are electrocuted will also be the last time.
You should always have mains voltage (220-240 AC in Singapore) between the L-in and Neutral. What you want to do is identify the Line in (always hot) wire and the neutral. With everything disconnected from the switches and the power on, there should be one wire with mains voltage measured between itself and the neutral. The other two wires will have near zero voltage to the neutral as they are the Line out wires to your light fixtures. Once you have correctly identified the L-in and L-out wires. TURN THE POWER OFF at the main panel and reconnect your switches.
Is it possible you don't really have a neutral in that box?
The neutral should be neutral everywhere and should have no effect. I can't tell much from the file you attached but if I am understanding you correctly you have two switches. One is a Sonoff that operates the light. One is a regular switch that operates the heater. If that is correct a typical wiring description is below.
You should have hot, neutral, and ground coming from the main panel.
The hot should connect to the L-in on the Sonoff AND to one of the switch terminals on the regular switch.
The L-out on the Sonoff should go to the hot side of your light.
The hot side of your heater circuit should connect to the other terminal on the regular switch.
The Neutral should connect to the N connection on the Sonoff AND the N side of your light AND the N side of your heater.
The ground connects to the ground terminals everywhere.
If there was no neutral in the box and you had to pull one then more than likely your hot wire runs from the main panel to the light fixture where it is then fed to the wall switch box, and switched to turn on the light. In other words two wires come from the light fixture to the switch box. One will always be hot and that is connected to the Sonoff L-in. The other wire would connect to L-out.
Did you fix it?