I did not believe you at first, but I tried it and you are correct. My quick answer to this is to use the Freeze Bar on your master sketch.
I don't think you should be able to edit sketch 1 while working on sketch 2, so this seems like a bug. My fix would seem to be more of a work around, but it should help you until this is figured out.
I just heard that you can also prevent this by turning off Instant 3D.
Well im glad its not me, and is just a bug or something thats been overlooked, ive never really known what "instant 3d" is, ill have to look into it.
Yes, Instant 3D makes this possible. One of the reasons that I turn off instant 3D and leave it off. It makes unintentional changes far too likely. Which could be a disaster if not noticed. I don't ever trust the CAD system to make design decisions. For the same reason, I tend to not use wizards. They make decisions that should be given thought by the designer.
Oh, and they are hard to light when wet.....