Thanks for your thoughtful response… and sorry it took so long to get back to you. I tried different combinations of capacitors, a diode on the load lines, etc… nothing worked. And then I put a 0.1uF capacitor directly between the power leads on the valve itself… and everything started working fine. Admittedly, I’m not 100% sure why… but I won’t complain :)

Thanks so much! It turns out that the relay (8x) module I’m using - https://www.amazon.com/gp/product/B00LX3UH9C - has isolated relays… but I’ll try a diode, too.