I don't know if this helps. When i was using Lineage 14 with selinux enforcing, i also ran into this problem. Developer also say that the ROM was not rooted but can be rooted if the user wants to root. Problem was Banking apps keeps rejecting. Finally, I downloaded and flashed "SUcleanup.zip." After that, the Banking app worked and stopped reporting that it detected "root." You can try searching for these files and try flashing them to see if it works for you. It seems that although Lineage roms does not come automatically rooted , it has some of the su binaries files and folders to make it helpful for someone who wants to root with chainfire's rooting method.