I thought the reboot from the diagnostic tool was more 'thorough'. I wouldn't have thought that Past Logs would've made it through.
Past logs do not get erased unless you reset (and don't restore) the hub. A diagnostic reboot should be the same as a "regular" reboot; it's just an option you can use if the admin UI won't load. Old past logs will get pruned when the size limit is reached, something like 256 KB unless that's changed recently.
Their presence, however, shouldn't be a problem. Do you have a specific issue you're trying to address?
No, just wondering.