rkhunter uses the contents of /etc/issue as the OSNAME for Leap 15.0.

-- This is 42.3 --
# grep 'OS:' /423/var/lib/rkhunter/db/rkhunter.dat
OS:openSUSE Leap 42.3
# cat /423/etc/os-release 
NAME="openSUSE Leap"
VERSION="42.3"
ID=opensuse
ID_LIKE="suse"
VERSION_ID="42.3"
PRETTY_NAME="openSUSE Leap 42.3"
ANSI_COLOR="0;32"
CPE_NAME="cpe:/o:opensuse:leap:42.3"
BUG_REPORT_URL="https://bugs.opensuse.org"
HOME_URL="https://www.opensuse.org/"
# ls -l /423/etc/os-release
-rw-r--r-- 1 root root 249 Aug  9 14:23 /423/etc/os-release

-- This is 15.0 --
# grep 'OS:' /var/lib/rkhunter/db/rkhunter.dat
OS:Welcome to \S - Kernel \r (\l).
# cat /etc/issue
Welcome to \S - Kernel \r (\l).

# cat /etc/os-release 
NAME="openSUSE Leap"
VERSION="15.0"
ID="opensuse-leap"
ID_LIKE="suse opensuse"
VERSION_ID="15.0"
PRETTY_NAME="openSUSE Leap 15.0"
ANSI_COLOR="0;32"
CPE_NAME="cpe:/o:opensuse:leap:15.0"
BUG_REPORT_URL="https://bugs.opensuse.org"
HOME_URL="https://www.opensuse.org/"
# ls -l /etc/os-release
lrwxrwxrwx 1 root root 21 May 16 03:58 /etc/os-release -> ../usr/lib/os-release