It looks like  C/C reconnect retains the database. It is correct behavior
However there is a problem that mysql_install_db.exe does not create
'test' database, thus using mysql_install_db to create datadir,
then restarting server and reconnecting does not succeed, if 'test' was
current db before reconnect.

The problem became apparent with MDEV-26715. Why it worked before,
with a simpler 'root' definition is a mystery.

Fix is to switch current connection to 'mysql' database, prior to
reconnect to the instance created via mysql_install_db.exe