Having a quick google it seems the systems may not always talk to each other that well. Maybe that's something that Star Alliance can work on cause it's a hassle and frequent flyers haaaaate hassle! It's fine if you do it once a year but running into those issues multiple times is a PITA. I guess you might get used to what you have to do but still, it's a pain!
I don't mind plugging in the other airline PNRs into the apps and have stuff come up but it's annoying when you don't get the other airline PNR. And then there is good old seating. It's a back end issue. I've had that before and no one picked it up but it got picked up at the airport and eventually a call to reservations on the starting airline to sort.

Sometimes it's easier with separate tickets if you have the time and you just work within the airline app. Changi/jewel easy enough to hang out at or head to somewhere close outside the airport for a quick bite/drink or shop for something.