When reroutes happen in IRROPS, it is really important to stay on top of the agent assisting you to make certain that these things are taken care of. It should have been possible for the agent handling your reroute to keep your seat and thus your meal, even if the ticket was reissued. It just takes time and everybody is in a hurry.

As noted above, a good chance your meal was physically on the aircraft. So, important to speak with cabin crew on boarding to ask that they locate the meal with your original seat assignment.