I'm not aware of it rubbing against anything unless it rubbed against the stem when forming. It definitely didn't touch anything hot. Hopefully the next one will not have any issues.
I agree it looks like damage while in bud - physical, chemical, heat, pest - all possible. Slight chance for dehydration induced spot calcium deficiency. Slighter chance for viral necrosis. In all cases, unless it repeats or there are more general widespread symptoms there is no cause for concern.