In vim 9.0.1046-1, pasting by holding the shift key and using either the middle mouse button or the "paste" entry from the context menu is broken at least when all of the following circumstances coincide:
- I'm using the testing repositories,
- vim is running inside tmux,
- tmux is configured with with "set-option -g mouse on" and "set-option -g set-clipboard on",
- I'm using GNOME terminal and GNOME is running in Wayland mode.

I have not tested if all of these are required to reproduce. By "broken" I mean that at least some stuff from the clipboard buffer is pasted, but also things that I had not copied (possibly the contents from the "non-shift" and the "shift" clipboard buffer in tmux get mixed/concatenated), and newlines and whitespaces/indentions get mangled/garbled.

To solve the issue, I've taken the PKGBUILD for the normal vim package(s), updated it to vim version 9.0.1157, and built the packages in a clean chroot using "testing-x86_64-build" from the devtools package. Updating to the resulting vim packages does indeed solve the issue for me.