Landlords and their managing employees, are odd sometimes. What happens is they have a rule, which their employees responsible have been ignoring, or been negligent in enforcing. They send down a message that the rule is being broken. The employees responsible then jump to attention and decide that , at least, they'll stop anyone else from breaking it, so the very next person who wants a dog is told they can't have one. That person is bound to ask them, or the employees feel bound to provide a reason to,'Why me?' The reason that they can come but up with is, of course, both on the spur of the moment and fatuous, but the real reason 'It is and always has been against the rules' is embarrassing because it's an admission that they haven't been doing their job properly over quite some time and worse, on repeated occasions, before.
You have been a victim of that, or something very, very close to it.
But having set themselves to enforce, the council won't budge. Your only answer is to feel hard done by; "Why me?"; but comply and either not have a dog or find a landlord who will allow you one.
(My own answer, in a very big block which had that rule and enforced it, was to buy a chihuahua, which I smuggled in and out in a bag, or under my coat. Never got challenged, but had I been and had I been threatened with eviction, the dog could have gone to my mother's. This is not an option open to all LOL)