We have a similar behavior. We can see the post being POSTed on the activity end-points and accepted but they are not shown in the different subscriber. The only issue we see (after a recent update of the plugin - we run 3.6.0.beta1-dev) Invalid date for ActivityPub post on topic.
What would be the best option for debugging this behavior? In previous version, we didn’t see that error and the posts were published on the subscribers properly.