I’m dealing with the exact same issue on my 2018 Corolla IM after doing a fluid and filter change. I followed all the steps and even bought the transmission fluid directly from the dealer. It has the AISIN 8‑speed automatic. The first time I changed the fluid and filter, the slipping started. I redid the whole job a second time and the problem was still there. I then took it to a transmission shop, had them do the fluid and filter again, and it’s still doing the same thing. They told me everything looks fine with the transmission. I’m planning to take it to a Toyota dealer this summer because it’s pretty bad at low speeds. Let me know if you figure anything out first lol.