This was a bug, if you are working from the source code repository it is now fixed in the latest code.
Problem is that FirstName and LastName fields were added to the main mp_User table and logic was added to map it there instead of in mp_UserProperties, but the mapping was present only for the Set not for the Get, so it was not retrieving the values.
Best solution is to get the latest code or wait for the next version before using FirstName and LastName as profile properties.
A workaround is to use different keys like FName and LName, but ideally you would want the data stored in mp_Users and this workaround will store it in the mp_UserProperties. So it is better to either get the latest code or wait for the next release.