Validation of Required Fields Across Multiple Tabs When Submitting a Form

Interface checks only the active tab because hidden tabs aren't loaded, so their validations don't run by default.
Create a single validation rule that checks each tab's data separately (local!tab1Data, local!tab2Data), then validate all at once when Submit is clicked.

The form layout also has a validation parameter. Add all your validations there.

To make this a good UX, you will also have to highlight the tab containing a failed validation, and add all the validations to the individual fields as well.

An additional and often-overlooked tool for this sort of general use case is form-level and section-level validations using a!validationMessage(), with the "validateAfter" parameter set to "SUBMIT".

Say you have 3 sections and you want to validate the contents of sections 1 and 2 only when the user attempts to click "SUBMIT" in Section 3; in this case you would need to manually maintain local variables tracking the validation of sections 1 and 2, and if any of them are not in a passing state upon trying to click submit in section 3, you would display your validationMessage (in whatever appropriate location you deem necessary), explaining that the user needs to review the items on section 1 and/or 2 as necessary.