Two obvious causes could be:
1) You do not actually commit the created object.
2) You commit the generalized object while looking for the specialized object. E.g. you look in the Administration.Account overview but in the microflow you create a System.User entity.
Two less obvious causes could be (but you would expect them to be cause an exception in the microflow):
1) Your random password does not follow the password policy of the project.
2) The user used to create new users is not allowed to create users with the specified user role.
Solved by migrating to 6.10
Did you check if the required fields are filled?