I've encountered an updating problem using yay.
lib32-libxpm is to be updated, but running yay -Syu ends up with:
==> Making package: lib32-libxpm 3.5.16-1 (Sun 02 Jul 2023 )
==> Retrieving sources...
-> Found libXpm-3.5.16.tar.xz
-> Found libXpm-3.5.16.tar.xz.sig
==> Validating source files with sha512sums...
libXpm-3.5.16.tar.xz ... Passed
libXpm-3.5.16.tar.xz.sig ... Skipped
==> Verifying source file signatures with gpg...
libXpm-3.5.16.tar.xz ... FAILED (unknown public key ACEB29740C9A4E97)
==> ERROR: One or more PGP signatures could not be verified!
-> error downloading sources: /home//.cache/yay/lib32-libxpm
context: exit status 1
:: (1/1) Parsing SRCINFO: lib32-libxpm
==> Making package: lib32-libxpm 3.5.16-1 (Sun 02 Jul 2023 ******)
==> Checking runtime dependencies...
==> Checking buildtime dependencies...
==> Retrieving sources...
-> Found libXpm-3.5.16.tar.xz
-> Found libXpm-3.5.16.tar.xz.sig
==> Validating source files with sha512sums...
libXpm-3.5.16.tar.xz ... Passed
libXpm-3.5.16.tar.xz.sig ... Skipped
==> Verifying source file signatures with gpg...
libXpm-3.5.16.tar.xz ... FAILED (unknown public key ACEB29740C9A4E97)
==> ERROR: One or more PGP signatures could not be verified!
-> error making: lib32-libxpm-exit status 1
-> Failed to install the following packages. Manual intervention is required:
lib32-libxpm - exit status 1
Manual intervention is required, but it doesn't tell you what you need to do to correct it.