I can’t see anything in the topic json. 
Could it be in the topic list instead? I think that’s where you see the ‘last activity’ for the topic. eg. when a topic picks up a spam post which then gets deleted, but the topic didn’t get a ‘reset bump date’ - the topic list would say something like ‘2 mins’ but when you went into the topic the only activity dates you see are the last post (which could be months/years earlier)