WMG / Kung Fu Panda: The Paws of Destiny

Po accidentally erased Lu Kang from existence during an off-screen adventure some time after Legends of Awesomeness.

Which would explain why a lynx is now Emperor instead of Lu Kang, and Ke-Pa, a Legends of Awesomeness exclusive character carried over to this series, at least in the form of a passing mention. Plus, Po's had plenty of adventures in the spirit plane and dealing with supernatural foes throughout the first series so it wouldn't be his first rodeo in having cosmic/time-space adventures. And during one of those adventures, he might've accidentally RetGonned Lu Kang as a side effect, without erasing his other adventures. Hence why Kang Zi is established as being Emperor for many years, with Xiao being next in line.