A test in Jaybird (org.firebirdsql.management.TestUserManager) adds a user through the services API and then retrieves the created user to compare the properties. When this test is run against Firebird 3.0 the username, firstname, middlename and lastname properties are padded with spaces (0x20) up to a length of 10 (no padding occurs if the actual value is longer). This only happens when the usermanager is Legacy_UserManager. The properties are not padded when the Srp manager is configured in firebird.conf. Padding also doesn't occur with Firebird 2.5 and earlier.
I am not sure if the problem is with creating or with listing the users, but it also occurs when the user has been created with a CREATE USER statement instead of the service API.
Currently tests on Jaybird use Legacy_UserManager because it doesn't support the new protocol yet.
Note: I have seen the behavior at least since 3.0 Alpha 2 (AFAIR it didn't occur with Alpha 1, but I am not 100%sure), but I didn't have time to investigate closely until today. I tested with the snapshot of today (184.108.40.206135).