calibre does not officially support Ubuntu either. If you use the installation method from
https://calibre-ebook.com/download_linux you will have the latest version, not installed via pacman. This is the only method Kovid supports.
Distros support calibre directly, but at their own pace. I'm generally quite fast at releasing updates. However, Manjaro is based on Arch, but is not Arch, and mainly what they do is delay the speed of updates under the rationale that "if there are problems, someone else will notice them first".
I don't acknowledge this as true, and would recommend instead using Arch, keeping an eye on the news / mailing list / forums if you wish.
If you *want* slower updates, I guess you could use
https://wiki.archlinux.org/index.php/Arch_Linux_Archive to pin your system to the monthly snapshots.