You shouldn't be seeing very many of these. Of course it's always possible that AWS is having a brief issue whereby we cannot reach the destination host, but it should be rare. In those cases, your webhook will go through our retry mechanism and eventually be delivered so these won't be lost. Looking at your response here I see that it attempted to take a full 5s to get a response from AWS (EventBridge) but will then timeout beyond that.
If you are continuously seeing this for your specific destination please DM your event source and we can try and loop AWS in for them to investigate.