If the person who signed it was nominated as the person to sign it, i.e. as representative of the OC, then that is fine. If the seal is on it, then that also matters, the use of the seal usually binds the OC. Usually when you enter into something like this there should be a resolution of the OC saying that it is resolved to enter into the agreement, certain person or persons are delegated to sign it on behalf of the OC, and that the common seal should be affixed.
Was there a resolution to that effect?
The common seal is usually kept by the strata manager, but they shouldn’t use it unless they are instructed to do so (in the absence of delegated powers).
If the agreement has been going for 8 years you would have difficulty disputing its validity because it has been on foot for so long, i.e. services under the agreement have been provided and paid for. That doesn’t stop you looking at appointing another agent, depending on what the termination clause says.