That Phoenix sun does a number on patience, trust me. Saw a similar thing years back where a guy used 48V coil relays on a 24V fire alarm loop, they'd test fine cold but drop out when the panel warmed up after lunch. The real kicker is bench testing with a variable supply, set it at the actual site voltage and let it run for an hour before you button things up. Also, those little sticker labels on the relay body are easy to misread when you're in a hurry, so I started double checking with a multimeter right at the coil terminals, not just trusting the box. Your callback story is the kind of thing that makes guys swear off cheap parts, but it's really about the 10 extra minutes of verification.