Here is another thought...sometimes - especially when the sunlight is shorter - and as you said yours are molting -- their egg shells are a bit thinner.What if the egg broke when it was stepped on or when it was laid? Then this hen could have just been an opportunist and not an egg eater.. ETA - she got caught with "egg on her face" - but may not even have broken the shell herself.
Since it didn't happen again, it might be a one-time thing. Boost their protein and calcium and keep an eye on her. Good Luck.!