I recall working on this not too long ago.
I believe it’s a simple regression because the markup changed a little bit with @techAPJ’s incredible work. This is totally expected as the entire functionality changed when the option to move to a PM was added 
So, the styles stopped applying since they were targeting an element that’s not there anymore. I pushed a small fix to update the selector and increase the width of that modal a little bit.
https://github.com/discourse/discourse/commit/b21266ed6f3178c9158584bf1986e4bb12923e1b
I think this is intentional and it matches the way we show them in the header.