In short, the problems stem from the lack of repo sync at INSTALL time. Manjaro differs from every other Arch distro in terms of what packages are available when you install software from the AUR.

Which is completely irrelevant because AUR "packages" are only very loosely related to Arch binaries. Your average AUR is just a source package developed by someone who most likely doesn't use Arch, plus a thin wrapper script that says "it needs these packages to compile and these packages to run".

As users of source based distros like Nix and Gentoo will show you, you can get a well-made source package to compile and run on an extremely wide variety of system states (and also distros, architectures etc.)

The fact that binaries on Manjaro are a few weeks late is completely irrelevant for something compiled from source from a reasonably recent source package.

You seem to be under the impression that AUR packagers perform extensive testing. They don't. They run it once, if it works for them they publish. They did that weeks or months or in some cases years ago compared to the time you install. By which time the relevance of that test to Arch or Manjaro or any Arch distro is tenuous at best.

There is one case where an AUR package can fail installing, and that's if the packager has requested a dependency in a version that for some reason isn't available on your system. This can happen to Manjaro due to the delay but also to any other Arch distro depending on whether the user is willing and able get that version at that particular time. Not everybody is willing to drop everything and update three times a day.

The other thing that people can't seem to get through their head is that AUR packages will break eventually as the system binaries are updated. You have to recompile AUR packages when they break. This is the same for all Arch distros.