I have the 19h1 preview running on a VM and noticed that, although I had disabled transparency effects in Windows 10, the logon screen would still get a transparent overlay (E.g. the background image became horribly blurred) when the logon name and password field appear.
I then double checked the transparency setting under colours and it was disabled.
I enabled it and disabled it again and now it seems to work. It looks like a minor bug – probably because it is a new setting, it gets defaulted to on and doesn’t follow the control panel setting.
A little annoying, but an easy fix.