Tabs disappearing should certainly be a rare occurence. But there are a couple of scenarios to be aware of - especially if you have multiple tills.
All Tabs need to be settled daily
Customer tabs in posBoss are not supposed to be kept open past cash up - and keeping them open can cause issues reconciling your stock to cash up and can even cause them to disappear as posBoss goes through it's cleaning processes.
If you have open tabs at the end of a day that can't be dealt with, you need to create an Account and assign the tab to that account before cash up.
Logging in to posBoss devices on more than one iPad
This is a very rare scenario but one that can occur. When you are logging in to the posBoss app on the iPad, be careful to log in to the correct till device. If you have multiple tills set up, you select the til to log into. If you select a till that is already logged in you will kick out the till. This can cause issues with data and syncing. If this happens please get in touch with posBoss support to help get your tills working and syncing properly again.