We would need a test case to understand what is happening in your situation.
It is possible to open the same page multiple times but you cannot rely on application items which are referenced in your page and change when changing tabs and it is your (the developers) responsibility to ensure session state is maintained. This is because application item values are only stored in session state and are not included in the page html and can't be submitted back to the server.
When submitting a form on a page all items are sent to the server and updated in session state. If you perform some other action on the page then you must ensure you submit all associated items with it to ensure session state is set correctly e.g. grid refresh, execute PLSQL Code etc. This is because the correct values are in the page/html but may not be correct in session state since the page you are on may not be the last page opened, so when you submit them with every AJAX action you can be then assured that session state will be updated to match what the page requires.
Therefore we always suggest that you use hidden page items on the page instead of application items e.g. for (manual) master detail relationships.