MacBook Pro 2017 running OS X 10.14.6 w/FileVault enabled.
Command entered by admin: sudo dscl . create /Users/ACCOUNTNAME IsHidden 1
1) Admin hides a user by running the command shown above
2) Admin logs out
3) User hidden in step 1 no longer appears on the login screen
4) OS X restarted from login screen
5) User hidden in step 1 is no longer hidden
An SMC reset has been performed, but it had no affect on this issue.