On timer "t" can fire with multiple picked instances if the timers happen to fire at the same time.
Seems like I'm not the only person who has encountered this issue before:
@Ashley Can you please add this to the manual for Timer? Given that Start Timer runs at a per-instance level to begin with, it is not intuitive that internally Construct groups coinciding times for the timer (which bears similarity to a trigger/callback such as "on collision") SOL On Timer event to include more than one instance if possible.
Knowing this would have saved me about an hour of debugging.