Tagging baggage over several flights has never been an alliance issue. It's been possible to do it way, way before the big alliances ever existed.

The matter of BP issue is more complex. It doesn't always matter about timings. I have been to Star check-ins where a BP for a connecting flight on another Star airline that was leaving the same country within 3 hours of the first check-in was impossible to issue. This has happened several times, in several countries with several different Star carriers.

All sorts of excuses are offered by check-in staff on why they could not issue the connecting BP.

I suspect the real reason is simply incompatibility of systems. The Star airlines can't even agree on a common booking reference when one ticket involves several Star carriers. How can they manage one BP issue?

Travel on Star is not totally seamless. It never has been. They should stop saying it is.