See the note about Python 3.12 in Testing Update 2024-05-02. The note itself isn’t as important as the fact that Python 3.12 is currently in the Testing branch.
Note that Manjaro’s Unstable branch is arguably on par with the Arch Stable branch. You will need to wait for it to reach Stable in Manjaro, which probably will not be too far away.
This will help you understand the Manjaro branch heirarchy:
Additionally, there has been much focus on getting Plasma 6 release-ready for Manjaro; this, in itself, has no doubt caused general delays as packages are prepared, tested, fixed, and tested again for compatibility and overall readiness for general availability. This tends to happen with every major update of KDE.
Cheers.