The ipaddrcheck does not validate that the start and end addresses of an IP range are logically ordered.
root@r14:/home/vyos# ipaddrcheck --verbose --is-ipv4-range 192.0.2.0-1.8.2.1 root@r14:/home/vyos# root@r14:/home/vyos# echo $? 0 root@r14:/home/vyos#
This issue remained hidden because most malformed ranges like 192.0.2.100-192.0.2.100 were detected correctly.
The problem was that the check didn't account for the fact that in_addr->s_addr is in the network by order (big endian), which makes comparisons incorrect for cases like in the task description.
Note: IPv6 is unaffected because the comparison logic is byte-by-byte there.