I am using the same account that I purchased the Supreme Edition on.
After making sure that my Windows computer was set to the proper time zone I synchronized it to internet time.
I have restarted my PC and booted up KI again.
According to my client, the season 2 and 3 characters are “available for purchase” even though it also recognizes that I have already purchased the Supreme edition.
To be more specific, after purchasing the Supreme Edition in-game, I did receive the season 1 characters, the 18000 KI gold, and the extra costumes. What I didn’t get were any of the season 2 or 3 characters. There is indeed another post in this forum that featured the same issue, although that one never got a response. You can see it here.