I have not worked with Apache MQ. But in general:
In a Publish/Subscribe model, when a message is published, all subscribers will receive that publication.
Acknowledgement by subscribers is only to messaging providers and not to the publisher. When a subscriber acknowledges the receipt of a message, that message is removed by the messaging provider so that the message is not delivered again.
If a new subscriber comes in after a message is published on a topic, then the subscriber will not get that publication. Any new publication on the topic will be received by the subscriber. Some providers like IBM WebSphere MQ have a feature called "Retain Publication" wherein a copy of a publication is cached. Any new subscribers who come in after a publication is made will receive this cached publication. Retain publication feature is useful when there is a time delay between publications on a topic, new subscribers will get the cached publication immediately rather than waiting for a new publication to be made by publisher.