KDE Plasma 6 made it to Arch about a week before Tumbleweed. Tumbleweed is also still using Xorg by default for Plasma 6. That said both had it in their repos withing 2 weeks of release. Is there some history here for Gnome on Arch?
These updates land on testing quickly, however due to the several packages updated at once, they all need to be tested by volunteers, and only when all of them are signed it's pushed out of testing