Yeah, what is weird is I'm doing it in the cave before the Kanto E4. The same place I was doing it when I hadn't finished Kanto/Johto. However, now that I have finished the entire story it seems to no longer be working as well as it did previously. I didn't see anything change in the changelogs, but that doesn't mean something didn't slip by or was forgotten. Please, let me know if you are having issues as well.
For reference: I am using level 100 Jumpluff. Toxic, flash, flash, leech seed, memento and swap in low level pokemon. I make sure toxic and leech seed hit. For example, currently, the level 50-53 Ursaring has enough hp to last another turn or two.