swaylock doesn't work with correct password. pam_authenticate Failed: Authentication Information Unavailable

[ser398]

When running swaylock on Arch Linux, I encounter an authentication failure with the following error:
2025-09-27 19:48:22 - [swaylock/pam.c:110] pam_authenticate failed: authentication information unavailable

Additionally, running pamtester swaylock <username> authenticate returns:
pamtester: Authentication service cannot retrieve authentication info

An strace of swaylock reveals the root cause as a permission issue:
openat(AT_FDCWD, "/etc/shadow", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)

The permissions of /etc/shadow are:
-rw-r----- 1 root root 1310 Jul 26 20:02 /etc/shadow

System Details

OS: Arch Linux
Kernel: 6.16.8-arch1-1
Swaylock Version: v1.8.3 (" DATE ", branch 'makepkg')
PAM Version: 1.7.1-1
Sway Version: 1.11

PAM Configuration

/etc/pam.d/swaylock:

#%PAM-1.0
auth      include   system-auth
account   include   system-auth
password  include   system-auth
session   include   system-auth

/etc/pam.d/system-auth:

#%PAM-1.0
auth       required                    pam_faillock.so      preauth
# Optionally use requisite above if you do not want to prompt for the password
# on locked accounts.
-auth      [success=2 default=ignore]  pam_systemd_home.so
auth       [success=1 default=bad]     pam_unix.so          try_first_pass nullok
auth       [default=die]               pam_faillock.so      authfail
auth       optional                    pam_permit.so
auth       required                    pam_env.so
auth       required                    pam_faillock.so      authsucc
# If you drop the above call to pam_faillock.so the lock will be done also
# on non-consecutive authentication failures.
-account   [success=1 default=ignore]  pam_systemd_home.so
account    required                    pam_unix.so
account    optional                    pam_permit.so
account    required                    pam_time.so
-password  [success=1 default=ignore]  pam_systemd_home.so
password   required                    pam_unix.so          try_first_pass nullok shadow
password   optional                    pam_permit.so
-session   optional                    pam_systemd_home.so
session    required                    pam_limits.so
session    required                    pam_unix.so
session    optional                    pam_permit.so

Expected Behavior

pamtester swaylock authenticate should successfully authenticate with the correct password.
swaylock should display the lock screen and unlock upon entering the correct password.

Actual Behavior

Authentication fails with the errors described above, preventing swaylock from functioning.