This happens for me from time to time, too.
At some point (and I don't know why), my crons stop working or hang and I have to disable an re-enable all of them one by one. After that, the xf_session_activity table gets cleared and everything works again. Is that maybe the same case for you?
Quick question:
I have two XenForo boards on the same server (different domains and db of course), for both I'm planning to use the following config:
$config['cache']['enabled'] = true;
$config['cache']['provider'] = 'SV\RedisCache\Redis';
$config['cache']['namespace'] = 'instanceX_'...