The old signal-desktop package had an improper line in the PKGBUILD. It was making a link which would only work if signal-desktop was previously installed while building the package. It is hard to say what the circumstances were that led to your issue while installing the upgraded package. I do know that installing the new package here fresh there is no issue if signal-desktop was not previously installed and uninstalling then come back and reinstalling the new package there is no issue. The package is and had been installed in /opt.
My changes to the PKGBUILD that added a .INSTALL file to add/remove the link and removed the line with the improper link:
+install="signal-desktop.install"
.....
- ln -s /opt/signal/signal-desktop "$pkgdir/usr/bin/signal-desktop"
The added signal-desktop.install file:
+post_install() {
+
+# Symlink bin command to /usr/bin
+
+ ln -s /opt/signal/signal-desktop /usr/bin/signal-desktop
+
+}
+
+pre_remove() {
+
+ rm -f /usr/bin/signal-desktop
+
+}