The miniExtensions Form usually only loads records from Airtable when the form is first loaded. Updates made in Airtable (or by other form users) while the form is left open in a user's browser will therefore not be taken into account at the time of submission (e.g. in field validation). There is one workaround for this, however:
When turning on the form's login functionality it will validate if the conditions set for being able to log in (e.g. correct status) are still met at the time of submission. This can be used to prevent multiple submissions by different users.
To take advantage of this feature, go to:
The Form's Share section
Make sure that How do users access their records? is set to Login page
Select the fields used to uniquely identify a record under Login Fields
Under Restrict Access, turn on Conditional Login
Add the required conditions, e.g. Status = Available
Now, the conditions (Status in this example) will be checked both when the user first loads the record (either by entering the login details, or by using the magic link) and when they hit the submit button. If the condition is not met in either of the two steps, an error message will be shown.