Issue with interconnected fire alarm

[cchn112]

Hi all. Looking to get some feedback regarding a fire alarm wiring issue in the house. The house was rewired a few years back by an electrician. When replacing some of the fire alarms over the weekend, I noticed that the interconnections were not working properly (testing one alarm did not set off the remaining in the series). Checking the sub-panel, I noticed that the fire alarm wiring goes into two separate 20 amp breakers (black and red wire). I have also tried removing all of the fire alarms and testing using 2 new fire alarms at connections nearest to the breaker. Each alarm would sound separately but not interconnected. I have tested continuity of the signal wire with a multi-meter and signal wire seems to be continuous. Not sure what would be the issue. I have posted a wiring diagram and a photo of the panel. Any comments and feedback would be appreciated!