Sadly they were the exact same Generic PLA and the exact same colour as I made sure of that before starting so like above for others it is not a great feature unfortunately.
I do not think if you are loading the filament from the external spool it will switch to the AMS when the filament runs out. This only applies to if you have the same rolls loaded in the AMS will the automatic switch over take place,
Sorry maybe I wasnt clear I had nothing on the external spool and no tube was going to anything as I only use the AMS but it switched to the External when the filament ran out on the first roll even though there was nothing there and has never been used.
Using the printers screen. Go to the filament configuration screen. Click on the external edit pen. On that pop up screen, there is a reset button to reset that slot to empty. Then the printer should not select that slot. Resynce your BS to the printer.
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Reviving this topic as it won’t let me start a new one 
With two identical spools and identical settings, auto switching enabled, it was showing the order of slot4->slot3 and everything was good. Started a second print and was surprised that now it started using the new spool in slot3 instead of continuing with slot4 which still has filament.
Also, I read about the bitter end of the filament being taped to the spool and the print stopping as it can’t feed it anymore. When the auto switch is selected, couldn’t it cut and unload it, then load from the other spool? Could this be a software enhancement?
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I will add my voice to the echoing thunder of Bambu users with valid points and logic!
Just ran into this, came here looking for a solution. AMS filament backup is still not working.
Need to be a bit more specific.
A lot of us have no issue with the auto-refill. That’s not saying you didn’t, just saying no one can help if all the info is “not working”.
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Two rolls of the exact same filament loaded, one ran out, and even with AMS filament backup checked, I still get “HMS_0700-2000-0002-0001: AMS1 Slot1 filament has run out.”
Printer?
I just wonder. I’ve used auto refill on my P1S without issue, on very large prints. I’ve even lied to it during that time and had no issue. Told it a 3rd party filament was BL matte and switched no issue.
Usual caveats apply. Update firmware, double check that filament has been updated in the slicer etc.