Apple is passing off this issue as "not an issue" as it seems to be affected by the underlying system. I do not know where else to go, but to report the problem here.
This issue was discovered in macOS where creating users with a period is permitted by the GUI (Users/Preferences as well as during the initial setup). Additionally, subsequent users created after the user that was created using with a period as a prefix to the username do not get a proper UID.
Attempts to reveal the hidden account do not work using `dscl . -list /Users` Additionally, attempts to find the account if the home directory is moved becomes very challenging. If macOS is build on the FreeBSD structure, the hidden account might be noticed by looking at the /var/db/dslocal/nodes/Default/Users directory (or FreeBSD equivalent).
No known patch/fix is available.
dscl is not a part of the FreeBSD base system. This seems to be macOS-specific utility so unfortunately we cannot really do anything about it.