Tab with table editor left open on a computer that was kept on overnight. When I returned to the table and tried to save it the save dots appeared but the table wasn’t actually saving. I got the warning that changes won’t be saved if I tried to close the table, when I right-clicked and opened ‘Save table’ in a new tab I saw the same table but without the more recent work. Saving the table as a new name also doesn’t work. It looks as though another user (jennifer) also had this problem but noone replied.
In order to be compliant with WordPress security requirements, save functionality of the tables are using a unique nonce value with a expire timer that will be used to compare with the value on the server side. In your case, that expiration time is elapsed and WordPress can not verify the authenticity of your save request. Only by reloading the page you can get a fresh security key. We are aware that it can cause issues like you are having, and because of that we are working on auto-save functionality that will save a backup of the table you are working on which will also refresh that security key. This functionality will be available in the future updates. Sorry for the inconvenience.