Zyra plants not choosing new target after previous target leaves range [gameplay]

I've been experiencing Zyra's plants not selecting new targets (or retargeting) after an enemy champion leaves range. For instance, plants will sit until expiration even when minions are present or if an enemy champion (re)enters their range. I typically notice this in mid lane as I'm farming and exchanging but it has happened while supporting (i.e. if the enemy adc leaves range, plants will not target the enemy support even if they are still within range.)

I'm not entirely sure if this is something built into her kit, seeing that it says they will prioritize the last enemy champion Zyra has attacked, but does that mean they cease fire altogether until Zyra attacks another enemy unit or champion? Or should they be reprioritizing automatically?

So in short, I'm not entirely sure if Zyra plant AI is wonky when it comes to retargeting, or if this is a matter of being used to automatic vs. stick shift. Any input or a simple answer would be much appreciated!