FileVault Encryption Issues On High Sierra (APFS)
If you have an encrypted drive before installing the OS you get that multiple login issue. If you do an install on a standard drive, then turn FileVault on, you won't have to double log in, as the user login will 'decrypt' the drive on its own.

I believe that's what solved a similar issue for me. I did a second clean install, this time formatting APFS (without encryption).