It's a client side problem rather than a server side problem. I have two computers that recently updated to Windows 10 2004 from 1909 and they're both experiencing this problem when previously they did not.
I'm lucky enough to have the access to the logs on the RADIUS server that performs the authentication for the L2TP VPN and it doesn't even process the authentication until after the password is entered. The same is also true for an old PPTP connection I was able to test with that's being hosted off of a Windows 2012 R2 RRAS server.