Transformerless power supply and relays.

[Yamin]

Hi,
I've got in a refrigeration system with a universal type relay module which became faulty. Its made up of 4x 12V relays and the power supply is a transformerless design (not a huge fan of this).
My first thought was why aren't there any 12V zener diodes? After some browsing on the web I came across this website:
https://www.homemade-circuits.com/cheap-yet-useful-transformerless-power/
Quoted from the site - "However, the 310V DC may be too high for any low voltage device except a relay. Therefore, an appropriately rated zener diode is used for shunting the 310V Dc into the desired lower value, such as 12 V, 5 V, 24 V etc, depending on the load specs."
Why is the zener diode omitted for relay circuits?
Troubleshooting the circuit I found out that the current limiting resistor R1 10 ohm resistors were out of value. I replaced them with the same Watt but 12 ohm resistors since I had them in my parts bin.
Another thing I noticed was that flyback diode across the relay coils were also omitted. So I added flyback diodes across the relay coil. Could there be a reason why the flyback diodes are not originally installed and could putting them in be detrimental?
The unit works now. I hope you could shed some light on the question I have.
Thanks in advance.
*I'm using an isolation transformer while troubleshooting this circuit.

[Kleinstein]

With a constant load (the realy always connected) there is no need for an extra limiting of the voltage. In some cases also the diodes at the rectifiers could be zeners and do the limiting.

[fzabkar]

When you interrupt the supply to the relay (by switching off the active terminal), the 4 rectifier diodes become the back-EMF suppressors for the relay coil.

[Seekonk]

Those resistors in series with capacitor often fail.  That is how I get a lot of free stuff.

[EHT]

I suppose the purpose of this circuit to make the relay work with 110 or 240v AC input, otherwise they could just have coils rated at the line voltage. The 2 caps might also be likely to fail..