Was printing from AMS Slot A2 Bambu PLA Matte with no Auto Refill set for that slot - spool ran out and printer switched to external spool configured with Bambu TPU with no message on Bambu Studio, Bambu Handy, or locally on printer. Since there was no filament actually in the external spool I would have expected that the print would pause but printer indicated that there was filament loaded and continued with job - printing in air -
I would log a support ticket and send in logs etrc @BryanH iff you havent already
I tried to simuilate this, with Bambu RFID PLA Basic Orange (all I have left in Babmu)
- I simply ācutā the filament at the spool level, and quickly taped the end to remaining filament on spool
- the spool motor kept running, the free remaining end of the filament got sucked down and eventually
- āno filamentā left and print job failed as I also didnt have a refill configured
- it also did not try and switch to any other AMS or External spools.
EDIT: In way of reverse test, I did confure an autio refil and it failed over from spool 1, to spool 2 - only spool 2 ass the was the only spool configured with the same Manufacture, type, colour and Pa\K value setup in AMs
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I think Bambu has work to do on Auto Refill. When I tried it last, it was set up with two black spools. One had about 250g and the other was full. I printed a model with the 250g spool, and it completed fine. The model was less than 30g. The next time I printed black, the printer grabbed the new spool even though I had the old spool selected. It seems to consider both spools as one with no real order of use. I turned Auto Refill off and will handle it myself. 
More than likely to be an error in filament selection on the āSend Printā screen when you printed the model. If you only clicked the send button on the app and then clicked send without verifying the filament chosen by the slicer, then it was a missed opportunity by you.
I know this because I made the same error myself before I figured it out.
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Logged a support ticket.
Was unable to reproduce when the slot ran out again. The only difference I noted was that I had cleared the external spool slot before the job.
Letās see what support sees in the logs.
Support got back to me right away - Jason is awesome -
logs said that the tool head filament sensor seems to have stopped working (continued to show filament present) - and that because the external spool had a filament type registered the job kept running -
Iāve tested the tool head filament sensor several times and it appears to be functioning normally - Iāve also done two tests where I ran out the spool in the AMS and the print paused -
My concern was shared with support that the printer switched to the external spool when the AMS slot ran out even when there was no auto-refill configured -
Thereās something off with the control logic here - AMS configured, no auto-refill set, AMS empty - printer continues job even though AMS registered empty? yes there is a tool head filament sensor but the system knows the odometer reading for the slot to the tool head (or does it?) and something isnāt getting checked.
Thoughts?
Support team is going to dive deeper into the logs - maybe thereās an actual bug - maybe not.
Had this happen when i didnt double check the Refill Order and also ensure in the print-send screen the correct āspoolā as selected as the first spool.
Great! Yes could be some niche, intermittant thing that you ran into. Good news.
As I recall, there was no way to differentiate between slots (spools). The print presented a circle with both spools and arrows in a circular fashion. It would grab which ever spool no matter what was selected. The model had the correct filament selected when sliced and sent. I shouldnāt have to mess with it. As I said, I think it needs work.
I guess the circlular thing could be made clearer, but the ā1stā spool used, from that circular list, is the one chosen at the print\send screen in the AMS slot, but if thats exactly what you did (souinds like it?) and it didnt work that way its likely some kind of bug. I did a double refil on a long black and red print, 2 x black, (Slot 1 and 3) and 2 x red (Slot 2 and 4) , and they both failed over correctly to their alernatiev spools, with the ā1stā spools as Slot 1 and 2.