Slot 1 is loaded with Bambu Labs Aero PLA. Slot 3 is loaded with support material for PLA. No issue loading and they are recognized correctly by the printer. I do a resync in Bambu studio and the results are the same as before the resync (and they are correct).
I have tried saving the slice, quitting Bambu Studio, turning off printer, turning on printer, loading Bambu Studio, and then replacing and end up at the same spot needing to select the slot that has Aero PLA.
Click "Slice Plate", then click "Print Plate". A pop up will appear where it shows your model, print time, and filament cost. That's where you select what slot in the ams you want to use for the print.
If you're talking about multi color printing, that's set up elsewhere. I forgot exactly as I don't have it right in front of me at the moment.