Fixedseed ignored by bitcoin-qt I play with bitcoin source code to learn,
I create my own fullnode, and change the chainparamsseeds.h to just use my own node as SeedSpec6, and comment the dns seeds in chainparams.cpp.
When open bitcoin-qt, it wont connect to node defined. but when I use addnode command, then it connect and start syncing.

Why same node can't be used form FixedSeednode config?

Thanks
Those nodes listed in chainparamsseeds.h are fallback seed nodes. They are not what Bitcoin Core uses by default to connect to the network, they are only used as a last resort if none of the other methods for finding nodes works.

Furthermore, the nodes listed there are seed nodes. This means that Bitcoin Core will connect to them briefly to get IP addresses of other nodes to connect to, and then disconnect from the seed node. They are not there to maintain full connections.

If you want to have a node connect to another node, use the addnode command.