The bug is simple and happens 50& of the time, so there should be no problem recreating it.
Basically, after the update, when you are in a lane with minions, if you spawn two plants and -hit- with the "Q" two champions at the same time, the plants will hit the minions and only sometimes, redirect their aim to the champions that were hit.
Step 1- make sure there are minions in lane.
Step 2- use both charges to spawn the plants.
Step 3- Use the Q to spawn the plants.
Step 4- Make sure the Q hits the wave and the two champions.
Step 5- Do not AA or use other spells.
The plants ignoring the -damaged- target did not occur when there was only one target except once, however, the plants did target the champion by themselves with minions in range after about 1.5 seconds.
The bug took place even in the ultimate once, where the number of targets was again, two.
The bug can be avoided by using AAs or the Root in most of the cases.