I have the default setting of
Ascending level then subject selected for my lesson ordering. Every once in a while I’m in the middle of lessons and accidentally refresh the page. Multiple times (including just yesterday) when I continue my lessons I have different lessons. I’ve not completed any additional reviews, so it’s not like more new items would have unlocked. As far as I understand, the ordering should be completely fixed/deterministic.
With all that said, I do have a script enabled that influences lessons, but I’m pretty sure it wouldn’t cause this. I’ll still try refreshing a handful of times with the script off, but I’m not sure if that will reproduce the issue since it happens inconsistently anyway.