Many people set up their browsers to auto-delete their history, clean their cache, remove cookies etc upon ending the session. Cleaning your cache happens to delete the location where your CoC/TiTS saves are stored. Even if your browser isn't set to do this, browsers are known to be finicky and will empty the cache occasionally anyway. Thus, all normal saves should be considered temporary at best. I recommend using Save to File as your primary save method.
Or, you may be using incognito or its equivalent. Incognito/private browsing sessions/whatever are garbage and break numerous things. They might seem decent on paper, but none of them are worth using.
