I agree that it can't be described as a bug. As it is an intentional limitation of substitution it can't be seen as a bug. Issuing and playing a substitution long term has never been the intended use of the substitution system. It is intended as a short term loan of the account whilst the 'owner' can't log in for personal reasons such as illness or vacation. Use for long periods was never the intended use. As such the limits are reasonable and not a bug.
If you wish to put forward the argument that replacement substitution should be able to do everything the owner can and it can be used on a long term basis then that is not the case against a bug but the case to revise the intended design.
