Mendel Save Thanks for your time! The offending plants will be the "New Species" nearby when you load in.
Things I've tried: Moving the plants works, but even moving and deleting them will just have them reappear in their new position. Naming the species doesn't prevent it from reappearing, and I backed up this save and started a new island, the issue persists on the new island. I can confirm that I used to be able to delete plants, so if there's just been a hiccup along the way and a fresh install is the answer, well there are far worse things.
Thank you very much for your time and responses :)