You have to k2 to BO 1 stitch, so after you k10, you have to k2 more and pass the first one of those over. Then you count how many sts you pass over, not how many you knit to get 10 bound off. The loop from the last st is the first stitch of the remaining stitches.
That’s easy to mess up. I always count the stitches I am supposed to leave after binding off a few to make sure I’ve done it right before I get too far.
AHA! So you really are knitting that 11th stitch and the 12th is the first one to get passed over. I think I have been doing this wrong but usually it’s not a huge deal if you are off by 1 stitch. I am now using a Fair Isle chart though and I could not for the life of me get that darn pattern to line up.