MarsSeed [1] filed an orphan request for pacdiffviewer [2]: Unmaintained since 2021. Reported broken and not compatible with the actual pacman/libalpm version since 2024-03-17 by AUR users.
One AUR user submitted a fix PR to upstreamand linked to it in AUR comment. That user also has a fork available that works. [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/pacdiffviewer/
