I actually can't think of a context where this would happen. There has been a recent upgrade of grub which could cause problems with dual boot systems, but it w

I actually can't think of a context where this would happen. There has been a recent upgrade of grub which could cause problems with dual boot systems, but it wasn't automatically upgraded to for precisely this reason (you have to go out of your way or do a fresh install). And a standard grub upgrade should have kept your previous settings intact. So - since this happened and I can say with some certainty that it wasn't by design - have you opened a bug report about it? (I know that the unfortunate tone of this specific bug report is unfortunate but most are handled in a friendly manner)