Observables, produced by the CollectionSelector, are merged. There is another operator rxcpp::observable<T,SourceType>::flat_map that works similar but concatenates the observables.
Just hit this issue myself; looks like there are several other functions with the same mistake (e.g. concat_map, concat_transform, flat_map and merge_transform).
Line in question:
Rx/v2/src/rxcpp/operators/rx-flat_map.hpp
Doxygen link: http://reactivex.io/RxCpp/classrxcpp_1_1observable.html#ab1195b083ffa5a97fb1deb854c1e9bee