Fried Voltage Regulator

I am trying to figure out why my photon fried itself. It has been controlling 3 solenoids for a sprinkler system at the end of our back yard and has been doing this fine for about 3 months. It suddenly stopped working/turning on. On inspection, one of the solder points of the voltage regulator (top left, beside VIN/GND) appears to have heated up and fried the contact leg of the regulator.
My question is, how can I prevent this from happening again? I’m using a cheap 12-5v step down converter off amazon. Is this the likely source or is it my wiring?

My setup is solar powered off a (very large and overkill) lead acid battery and 40w solar cell. I am using a solar charge controller for this. Multimeter showing a charged battery at 13V. Showing about 1.5V off the 12V side of the controller, but this is without any load.

Below is a simplified diagram of what I’ve made. My big question is at the bottom left, where the 5v and 12v are all on a common ground. I’m thinking this is maybe what did it? I did not realize I did this until just now. If that were going to be an issue, why would it not arise until months later?

I would add flyback diodes to prevent a large surge of current when the power is removed from the solenoids.