Splitting trains is a quite common thing in Germany (though more long distance) and communicated in the official app.
If third party apps don't show that information that's on their part. Usually it's also said after departure inside the train by the conductor, though maybe just on long distance trains.
They still get it wrong quite often. Worst case is when the train arrives in reverse cart order, and the carts are labeled wrong. Bonus points if your reserved seat is in a cart that's missing.
Yes, although quite often they forget not everyone speaks German.
I once had a bit of Schadenfrunde while travelling in Netherlands, having the conductor telling us to switch trains in Dutch, and all my German fellow travellers wondering what it was all about.
