x86_64 works fine for me too. It is the ARM package that I am having issues with. I too am on arm-unstable but I tried it on arm-stable and had similar results. It could be a keyring issue. But it is weird that I can use pacman -U and it installs fine. It is with pacman -S that it fails.
debug: checkdeps: package xsel-1.2.0.20190821-1
debug: returning error 33 from alpm_db_get_pkg (../lib/libalpm/db.c: 248) : could not find or read package
debug: setting download size 18660 for pkg xsel
ls -l /var/cache/pacman/pkg/xsel*
-rw-r--r-- 1 root root 18660 Mar 11 2020 /var/cache/pacman/pkg/xsel-1.2.0.20190821-1-x86_64.pkg.tar.zst
As you can see, the file size that pacman sets to download matches the actual file size. It is incorrect in the arm repo, this is the cause of the error.
This would depend on what the ARCH ARM community repo has for a file size. The package is fine, the issue is what is reported and stored for the file size in /var/lib/pacman/sync/community.db.
@Strit I just noticed that the Arch ARM repo has xsel-1.2.0.20200527-1 as does the Arch repo. This is a newer package than what is in the Manjaro repos.
This mirror appears to have performed a sync 16 minutes ago… still no update to the xsel package.
Edit: But I see the update made it to the x86_64 mirror… maybe it just needs more time.
Edit 2: Note to self… It seems the Manjaro mirrors sync against a snapshot of the Arch ARM repo. It does not matter when the repo syncs, it matters when the rsync snapshot was taken that the mirrors sync against.