I also think this is a bug, but while we wait for an official response, I’ve managed to work around it.
Technically, this only happens if you’re using the Nouveau template, legacy still works fine.
There are two steps to fix the issue. Firstly, in your theme folder create the folder(s) buddypress/common/js-templates/messages and inside messages, create index.php.
Into index.php here, copy the contents of the buddypress plugin file (within the plugin folder) bp-templates/buddypress/common/js-templates/messages/index.
Inside the newly copied index.php file, go to line 37, where the input named “send_to” is located and change it to this:
Firstly, the link being generated on for the user list view has “…#compose?r=…”, which is meant to use the compose view as a backbone view I presume, but that ends up not switching to the compose at all, so switching out # for / makes it go to the correct location.
Secondly, the input field has no code (PHP or Javascript within the Backbone engine) that should take the query variable and inject it as the recipients username. The code itself does exist (bp.Nouveau.getLinkParams), I just don’t know why it wasn’t implemented here somehow.