Well, it hasn’t reached Testing branch yet, and may even be completely skipped over in favour of Plasma 6.5:
You can check which version of Plasma each branch is currently on here: Branch compare for Manjaro - plasma-desktop
If you need Plasma 6.4 urgently, you can easily switch to Unstable branch:
To switch to Unstable branch:
sudo pacman-mirrors --api --set-branch unstable
After you changed the branch, rebuild the mirrorlist and update your packages:
sudo pacman-mirrors --continent && sudo pacman -Syu