Related code

static int
pam_usertype_is_system(pam_handle_t *pamh, uid_t uid)
{
    uid_t uid_min;
    uid_t sys_min;
    uid_t sys_max;

    if (uid == (uid_t)-1) {
        pam_syslog(pamh, LOG_WARNING, "invalid uid");
        return PAM_USER_UNKNOWN;
    }

    if (uid <= 99) {
        /* Reserved. */
        return PAM_SUCCESS;
    }

    if (uid == PAM_USERTYPE_OVERFLOW_UID) {
        /* nobody */
        return PAM_SUCCESS;
    }

    uid_min = pam_usertype_get_id(pamh, "UID_MIN", PAM_USERTYPE_UIDMIN);
    sys_min = pam_usertype_get_id(pamh, "SYS_UID_MIN", PAM_USERTYPE_SYSUIDMIN);
    sys_max = pam_usertype_get_id(pamh, "SYS_UID_MAX", uid_min - 1);

    return uid >= sys_min && uid <= sys_max ? PAM_SUCCESS : PAM_AUTH_ERR;
}

static int
pam_usertype_is_regular(pam_handle_t *pamh, uid_t uid)
{
    int ret;

    ret = pam_usertype_is_system(pamh, uid);
    switch (ret) {
    case PAM_SUCCESS:
        return PAM_AUTH_ERR;
    case PAM_USER_UNKNOWN:
        return PAM_USER_UNKNOWN;
    default:
        return PAM_SUCCESS;
    }
}


===================================================================

So according to the code, the followings UID are system users:
  * UID<=99
  * UID is between SYS_UID_MIN and SYS_UID_MAX
  * Kernel overflow uid: default is 65534

And for regular users:
  UID not in the system users except (uid_t)-1)

I agree with Martin here. All the UIDs <= SYS_UID_MAX should be treated as system. I'd simply drop the check of SYS_UID_MIN.

Although maybe there should be also some more sanity checking logic to verify that the uid is < UID_MIN.

I.e., uid <= SYS_UID_MAX && uid < UID_MIN

master:
    pam_usertype: only use SYS_UID_MAX for system users - 370064ef6f99581b08d473a42bb3417d5dda3e4e

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (pam bug fix and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2022:7723