It seems like raw aluminum ore will sometimes randomly smelt directly into a plate instead of into Alumina? But this is not indicated anywhere in the recipe. I'm not entirely sure if this is a Factorio limitation for why it can't be shown in the recipe, but maybe add it to the description of the Alumina recipe? This doesn't seem to be made clear anywhere. Maybe a little section in InformaTron could also be nice to have (this was really weird and took me a few minutes to figure out why I was having aluminum plate shortages and seeing plates on my Alumina belts lol).

With my mods alone this should not happen. I see two most likely possibilities.

1. Another mod has changed this.
2. There's an inserter that's accidentally putting alumina into a furnace that's only supposed to take the raw ore.

Hope this helps, but if not, I might need to take a look at a save file.

oh, weird, maybe i somehow had alumina circling back onto the ore belt then? but i have no idea how i accomplished that since at some point before i was using a belt for the ore i was using direct insertion from burner miners into stone furnaces and it was happening then too. now i cant replicate it in a creative world and it was too long ago for me to remember what i did, so i'll just assume i was somehow doing something weird then lol