Caulking bathtub, is there supposed to be a hole?
What it looks like you are getting out of the gap is the old grout and the caulk holding onto the grout. The grout when it was applied was the finish on the surface, but then the house dried out since it was new construction and caused some of the grout to loosen, so it was caulked at a later time. You are simply removing something that needs to be removed anyway. Clean it out very well, getting the old soap scum out of the gap so the new material will seal to the tub and wall and not to the soap scum. Let it dry thoroughly and caulk.