My playbook:
Hi there,
My understanding is that when ansible makes a connection to the remote machine it it’ll make the connection as ‘ansible_user’, and doesn’t re-evaluate it for the play. I’ve split it into two plays, I think it does what you want. Sorry for the formatting 
Running the playbook with -vvv shows the first play connecting as my default user (ansible) and the second play connects as user boblah.
Ansible 2.9