Same problem here.
And: I won’t get an ajax recommendation by entering @username in the field with bp-nouveau.
It seems to be a problem with the “bp-nouveau” theme.
With the old standard-theme, it works!
BuddyPress 3.2.0
WordPress 4.9.8
I had this issue too. I just switched from the new templates back to Buddypress legacy and it is now working to click the private message button and you can start composing the message to the user immediately. It is something in the buddypress nouveau template.
I think this is an important core-bug, because the bp-nouveau is the new recommended theme and this leads to disrupt the core messaging function.
Won’t anybody of the developers correct this?
How to install the patch please?