Nextcloud server 12.0.5
Debian 9.3 4.9.0-4-amd64 #1 SMP Debian 4.9.65-3+deb9u1 ( upgrade not done yet )
/ apache 2.4.25 / php 7.0.27 / 10.1.26-MariaDB / mysqlnd 5.0.12 / phpadmin 4.6.6deb4 / Redis server v=3.2.6 / HTTP2 /
User created as Benjamin.
Log-in as Benjamin OR benjamin the same.
Bug or features ??
Nextcloud server 12.0.5 --> 13.0.0 no problems so far
based A8-6500 8GO RAID6 8*6TO
4.9.0-5-amd64 #1 SMP Debian 4.9.65-3+deb9u2 (2018-01-04)
/ apache 2.4.25 --> 2.4.29 / php 7.0.27 / 10.1.26-MariaDB / mysqlnd 5.0.12
/ phpadmin 4.6.6deb4 / Redis server v=3.2.6 / HTTP2 /
OK, tiny vs capital bug/feature is also present in NxC13.0.0 …
So, i guess it is a feature. 
To be honest, that “feature” is a non sense for any user with a little linux/devell knowledge as those system are case sensitive.
User=uSer=USer=usER=UseR=… 