I have a peculiar problem with a PCB. This has an esp12f attached to two relays. The PCB has ground plane and is working fine except when I connect a 220v AC ceiling fan through a speed regulator. When I do that, I get abrupt restarts of esp12f. I have an RC snubber with CB221 capacitor and 390ohm resitance across the load which has improved the stability of the system. I already have a 1000uf capacitor across power pins of esp12f. I found one more thing. If I use different AC lines/phases to power the esp12f 5v power supply and the ceiling fan power supply, then there is no such problems. But I cann’t do that as I need th esp12f and fan to work with my inverter backed up line. what could be the issue.

The firmware is tasmota which shows hardware watchdog as the reason for restarts.

 

I found the problem. It was the back emf from the fan speed regulator that was causing the interference. I solved it by using a RC snubber across the hot contacts( COM and NO) of the relay. The Capacitor used is 224J250V which is 0.22 uf capacitance and resistor is 100ohms. But in your case you may have to find the resistor and capacitor through trial and error.
More info and good help can be found from https://www.homemade-circuits.com/prevent-relay-arcing-using-rc-snubber-circuits/