When a Server (DNS Name) has multiple IPs (IPv4+IPv6 or multiple for better availability) Minecraft only tries connecting to the first IP returned by the DNS API call.

If that one IP is unavailable Minecraft will say that the server is down, even though it is available through other IPs.

It therefore also doesn't use the best (fastest responding) server.

Example:

"a.example.com" with:

• 192.0. 2.146
• 2001:db8:2::2
• 2001:db8:1::2

When a user now tries to connect to this server Minecraft will only use the first IP the DNS client returned and ignore all others. And if 192.0.2.146 is unavailable or has degraded performance (because of a CGNAT on the user's side) connecting will fail and users will have a worse experience playing on the server.

The correct behavior is documented in RFC1123 (Section 2.3 Applications on Multihomed hosts) https://www.rfc-editor.org/rfc/rfc1123#page-14 and RFC8305 https://www.rfc-editor.org/rfc/rfc8305