The matron's skill was about 80 or so, though I don't know exactly what the skill level does - it may very well be the percent chance that the matron acts correctly. Neither girl had the fragile trait and both had a con of 100. The statistical probability of the matron failing to take both of them off duty at the same time in enough consecutive turns to kill them both is exceedingly low.
Assume that skill level = % chance of acting, and a matron skill of 80.
Turn 1, both girls are tired enough to trigger the matron, but she fails to trigger both times (20% chance per event, combined chance is 4%)
Turn 2, girls health declines, matron fails to trigger on both (again, 4% chance of occurring, total chance of event = 0.16%)
Turn 3, ditto (combined event chance = 0.0064%)
Turn 4, ditto (combined event chance = 0.000256%)
Turn 5, dead if matron doesn't act (combined chance = 0.00001024%)
The chance of the matron failing to take 2 girls off duty enough consecutive times that they both die at the same time is a tad bit more than 1 one-hundred-thousandth of 1% (odds are 9,765,625 to 1 against). I feel far more confident labeling it a bug than random happenstance.