Unusual behaviour when connected to light switch

Hi, hoping someone is able to help me. I connected my Shelly 1 up to a single on/off toggle light switch today so that it can switch a lighting circuit remotely in addition to retaining the existing switch. When the Shelly is switched via the app the lights just flash on and off repeatedly. There seems to be no way of keeping them permanently on. I have checked the wiring to the Shelly and it all appears to be as per the various wiring diagrams. I’m going to test it in another location to check it’s not the wiring but does anyone have any suggestions what the problem might be? Many thanks

By both. The physical switch allows me to turn it off but never on without them flashing. I’ve just tested the Shelly on another circuit and it works fine so the unit appears to be ok and the problem seems to be with this circuit. There is definitely a neutral connected to the Shelly as otherwise it wouldn’t even power up to connect to the app.

Interestingly, the bulbs on the circuit also seem to flicker at high frequency (whilst also turning on and off at low frequency!). The high frequency flicker can be resolved by changing one of the LED bulbs to a halogen one. Could the load on the relay side of the circuit be an issue?

I can’t upload a photo of the connections at the moment as it is currently disconnected but I can draw a diagram if needed.

So it turns out the wire I thought was a neutral (and thought I’d tested as a neutral) is actually another permanent live, which explains the unusual behaviour! It also sadly means I have no neutral at this switch and therefore cannot use the Shelly. The lights are fed from a junction box and have no permanent live so I can’t really connect it that way either. One day I may discover where the junction box is but until that day I will park this project for the time being.