I am unsure whether this is a bug, or an update that i may have missed, but i play Zyra as a midlaner often, and after the most recent patch i noticed that her plants would randomly stop attacking the closest minion, or would prioritize minions over champions that were well within range, even when i was also auto-attacking the champion.
This has been witnessed on plants spawned with both her Q and E. They sometimes pause for up to 2 seconds before they continue attacking, and they usually change targets after this pause. (I noticed it hit a melee minion once, then after a pause switch to a siege minion while the melee one was still alive).
When the plants spawn, the attack speed is normal, then it becomes delayed after they reach around 2/3 - 1/2 on the timer.
Unsure how to recreate this, as i have only played one game like this, but i will try it in a separate game to see if the same results are obtained.