I don't know if anyone else has expirenced this behaviour as well.
I had a fresh installed Windows server 2016, and installed the WFA 4.2 build (WFA-x64-V18.104.22.168.1-B4759965)
The installation was successful but the predefined account "admin" was not working when trying to access the WFA instance.
I noticed that the use was never created in the table user within the database wfa.
I tried several ways to install it, but always came up with the same situation.
The only hint i have is that i changed to a admin password that was less complex then pwgen gave me with the specific options i had in the CLI command.