The heimer turrets have always been wonky, and there is a minor bug with their target acquisition that has been around for the better part of a year now where they try to attack something outside their range. However, since the last patch, heimer turrets will often sit there not attacking anything at all, even with his ult up and getting hit by champs right next to them. I've noticed a large number of bug reports in regards to zyra's plants and can't help but think some pet AI got messed up somewhere along the line.