New user state updates sometimes aren't processed correctly

from irctest CI:

1740592607.789 1: connects to server.
1740592607.789 2: connects to server.
1740592607.789 2 -> S: NICK sender
1740592607.789 2 -> S: USER username * * :Realname
1740592607.797 S -> 2: :My.Little.Server NOTICE * :*** Looking up your hostname
1740592607.798 S -> 2: :My.Little.Server NOTICE * :*** Found your hostname: localhost
1740592607.798 S -> 2: :My.Little.Server 001 sender :Welcome to the test Internet Relay Chat network, sender
1740592607.798 S -> 2: :My.Little.Server 002 sender :Your host is My.Little.Server, running version sable-0.1.0-15f35c80a72c71a4796c5a2e04055817d989c3cb
1740592607.798 S -> 2: :My.Little.Server 003 sender :This server was created 2025-02-26 17:56:47.797522657 UTC
1740592607.798 S -> 2: :My.Little.Server 004 sender My.Little.Server sable-0.1.0-15f35c80a72c71a4796c5a2e04055817d989c3cb ioZ ntsim bqeIkov
1740592607.798 S -> 2: :My.Little.Server 005 sender EXCEPTS INVEX FNC UTF8ONLY MONITOR=64 CASEMAPPING=ascii CLIENTTAGDENY=* HOSTLEN=64 NICKLEN=15 USERLEN=10 CHANMODES=bqeI,k,,ntsim MSGREFTYPES=timestamp PREFIX=(ov)@+ :are supported by this server
1740592607.798 S -> 2: :My.Little.Server 422 sender :MOTD File is missing
1740592607.798 S -> 2: :sender!username@localhost MODE sender +
1740592607.798 S -> 2: :My.Little.Server NOTICE sender :The network is currently running in debug mode. Do not send any sensitive information such as passwords.
1740592607.798 2 -> S: PING foo
1740592608.604 2 -> S: PING synchronize414.391973892
1740592608.604 S -> 2: :My.Little.Server PONG My.Little.Server :foo
1740592608.605 S -> 2: :My.Little.Server PONG My.Little.Server :synchronize414.391973892
1740592608.605 1 -> S: CAP LS 302
1740592608.605 S -> 1: :My.Little.Server NOTICE * :*** Looking up your hostname
1740592608.605 S -> 1: :My.Little.Server CAP * LS :message-tags server-time echo-message batch labeled-response userhost-in-names away-notify account-tag multi-prefix draft/chathistory sable.libera.chat/persistent-session draft/account-registration draft/channel-rename
1740592608.606 1 -> S: CAP REQ :echo-message server-time
1740592608.606 1 -> S: nick bar
1740592608.606 1 -> S: user user 0 * realname
1740592608.606 1 -> S: CAP END
1740592608.607 S -> 1: :My.Little.Server CAP * ACK :echo-message server-time
[nothing happens]
1740592787.743 S -> 1: ERROR :Registration timed out

Start by tracing the registration and CAP negotiation paths implicated by the irctest CI log, where client 1 receives CAP ACK but no further response. Reproduce the trace and identify why registration remains incomplete; done means the client proceeds past capability negotiation without timing out.