Nothing new here. BokuBen did it few years ago and it wasn't just a few flimsy extra chapters but proper stories that lasted several full chapters.
Open ending is as good as harem ending or one girl winning ending, if done properly. It should be an ending with open question "who won?" and hints for every girl so there's no clear winner but there's equal probability that it could be every girl. This allows readers simply answer this question for themselves.
This manga did it wrong, because it ended with definitive status quo (that lasted even after time skip). So it's "nobody won" while it should be "it could be either of them" and that's the problem.