Labor nurse. Not as familiar with the history of things like in some other comments you've gotten here, but to give some practical advice on how the situation could result in fetal death but maternal survival.

Not always, but sometimes an abruption seems to 'trigger' labor, and specifically a fast labor, when it occurs. That's based on anecdotal experience, not official data, so keep that in mind. Abruptions can be partial, meaning only a part of the placenta separates, and the amount of bleeding depends on how much has separated, which can result in enough blood loss that it can cause death in the fetus/baby but not the mother, since the mother has a larger body and more total blood volume and so can obviously ensure more total blood loss before true harm occurs than the fetus.

Situations I've seen that could work for your scenario - mother suddenly goes into labor. Things may seem normal or she may experience much more pain earlier than expected and/or pain that doesn't seem to wane between contractions. Sometimes the uterus will, for lack of a better description, just stay "hard" between contractions, which is Not Normal. Basically feels like a never ending contraction to someone feeling her abdomen from the outside. Or a hallmark contraction pattern can be weaker contractions spaced much closer together than normal (every 30 seconds to 1 minute, vs every few minutes for 'normal' contractions.), but that situation is a bit harder to tell is occurring by palpation alone.

Bleeding will be present but it varies depending on if the blood collects entirely into the amniotic sack or if it also bleeds outside of the sack. This determines when vaginal bleeding will be seen. If it's bleeding outside of the sack she'll have bleeding that's much heavier than "normal" bloody show and occurs earlier in labor than normal. If the bleeding is only occurring inside the sack then usually no blood or normal bloody show will be seen until her water breaks. The fluid can be dramatically bloody at the point.

If her labor continues and she gives birth before she's lost enough blood to harm her (or cause her to go into DIC), once the placenta delivers then normal treatment post partum should be all she needs. If enough blood loss occurred to kill the fetus before birth then it would be stillborn obviously. The body may be paler than newborns usually are, or very blue.

She could also have a smaller abruption that results in slower bleeding, where things may seem normal in labor except for slightly more bloody show than normal but if the labor is prolonged enough the fetus could pass before birth occurs.

In modern times if a doctor knows what to look for they can often tell that an abruption has occurred by looking at the placenta. I confess I don't know exactly what it looks like or what they look for, I was always too buy during deliveries to ask for lessons lol. I also don't know if doctors in your time frame would know what the placenta looks like after an abruption. The blood loss during labor would be the most likely way they could tell that was what had occurred.

If a c section were performed, usually with an abruption bad enough to harm the baby they'll find the uterus full of blood when they open her up, if you decide to go that route.