Some systems report IPv6 as supported (as in, getaddrinfo() returns both AF_INET6 and AF_INET addrinfo's), but an attempt to create an AF_INET6 socket fails, because IPv6 support is either not fully available or disabled.
This does not prevent the server from starting up, because activate_tcp_port() falls back to AF_INET if creating an AF_INET6 socket fails. However, the "Failed to create a socket for IPv6" error is printed in the error log, which makes all MTR tests on such systems fail with the following symptoms:
This is a regression introduced by the patch for
MDEV-4379, as before that change active_tcp_port() always tried creating an AF_INET socket before AF_INET6.
A similar problem has been fixed in MySQL with https://github.com/mysql/mysql-server/commit/da66e3850d0b186a3c86e06daa4bc3208ad5cc62