Watching the replay, it looks like some of the engines failed to light during boostback reignition. Then, either total flameout occurred, or AFTS started cutting fuel in preparation for termination (maybe there are settings for "terminate right tf now" and "try to shut down engines before popping off", idk just speculating).
Either way, it looks like the start of boostback was not quite norminal, and AFTS decided that wasn't close enough to the flight envelope and decided to exit status 1.
Some are speculating that the flip maneuver sloshed the fuel too much and resulted in vapor ingestion and/or complete fuel starvation. The fact the failures are clustered on the side the fuel would slosh away from adds weight to this idea.