LocalDB v14 creates wrong path for mdf files
UPDATE

As of CU 6 for SQL Server 2017, this bug has been fixed. It is now possible to execute the following successfully:

CREATE DATABASE [CreateDatabaseTest];
DROP DATABASE [CreateDatabaseTest];


The problem, and the fact that it is fixed in CU6, is documented in the following KB article:
FIX: "Access is denied" error when you try to create a database in SQL Server 2017 Express LocalDB

To get the Cumulative Update, please go to the following page and grab the top (i.e. latest) build, which might be newer than CU6 depending on when you see this:

SQL Server 2017 build versions



BELOW INFO OBSOLETE AS OF SQL SERVER 2017 CU6 (Released 2018-04-17)

The lack of a backslash in the combined Path + File name appears to be a bug with SQL Server 2017. I just ran into it myself. I even tried editing the Registry to add a DefaultData string Value for C:\Users\MyAccountName\ in both of the following Keys (the 3 default paths are not in any of the LocalDB registry keys that I looked through):


Computer\HKEY_CURRENT_USER\Software\Microsoft\Microsoft SQL Server\UserInstances\{some-GUID-value}
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL14E.LOCALDB\MSSQLServer


And yes, I did shutdown and start up again the LocalDB instance in both attempts.

However, I am not convinced that not being able to change the default paths is a bug as it might just be poor documentation and poor error handling combined. I say this because I just tried editing the default locations for SQL Server LocalDB versions 2014, 2016, and 2017, and all resulted in the exact same error, which in itself is odd due to being from RegCreateKeyEx(), which should be dealing with the Registry and not the file system.

Not being able to change the path is unfortunate due to the lack of backslash when creating a new Database without specifying the files to use. However, I was able to create a new Database using the full CREATE DATABASE syntax as follows:

CREATE DATABASE [XXXXX]
 CONTAINMENT = NONE
 ON PRIMARY 
( NAME = N'XXXXX_sys', FILENAME = N'C:\Users\MyAccountName\XXXXX_sys.mdf',
  SIZE = 8192KB , MAXSIZE = UNLIMITED, FILEGROWTH = 65536KB ), 
 FILEGROUP [Tables] DEFAULT
( NAME = N'XXXXX_data', FILENAME = N'C:\Users\MyAccountName\XXXXX_data.ndf',
  SIZE = 8192KB , MAXSIZE = UNLIMITED, FILEGROWTH = 65536KB )
 LOG ON 
( NAME = N'XXXXX_log', FILENAME = N'C:\Users\MyAccountName\XXXXX_log.ldf',
  SIZE = 8192KB , MAXSIZE = UNLIMITED, FILEGROWTH = 65536KB )
 COLLATE Latin1_General_100_CS_AS_KS_WS_SC;
GO