Arc Fault Breaker trips when 2 specific lights are turned on
Arc Fault breakers detect -- wait for it -- arc faults.  A connection is arcing.  The #1 reason for that is use of backstab connections, but hold on - you're using #12 wire, and backstabs are not made to work with #12 wire.  Well, if someone insisted, that might do it.

Regardless, 99% of wiring problems are at terminations, so that is the place to look.