I have run into this exact same issue, and it seemed to resolve itself over time, but I had a string of these for a few months. My best guess is that there was a situation that caused the cigars to dry out slightly (not to the point of damage), and then they are getting rehumidified from the outside in. This allows the inside of the cigar to burn faster than the outside, where the ember can enter into the cigar beyond the burn-line, and create a hot and hard charcoal-ish spot.