Wired fire/smoke alarms not communicating with each other
It turned out to be a short-circuit. The cable connecting one of the alarms in the basement had been trapped by the electrical box cover. The cover is metallic, and had cut into the plastic enough to make contact. 

To find the issue I turned off the breaker, disconnected the signal cable from all of them, and started testing them from the top of the house using a 9V battery, as mentioned in one of the comments (+ to orange, - to white). Started with two, verified it was working, and kept going connecting alarms to be signal cable until I found out the culprit. After detailed inspection of the one that was causing the issue I noticed the cable damage.

Thank you @ThreePhaseEel for the 9V battery trick. It was really useful.