[Joel]

That large locked rectangle has become a line label for the line running up to the smaller box.

Any shape can be dragged directly onto a line to convert it to a line label, which is what I think has happened here accidentally.

So, regardless of its locked state, it moves around as the line moves around.

Think I'll muck around with locking line labels, etc. and at least file a bug about that.

Thanks for pointing this out, and I hope the above information helps.