You could argue that this is a bug, and Mendix should correctly handle validation for fields that are hidden. So you could file a ticket for this @ http://support.mendix.com/
As a workaround,if you are triggering the validation in a microflow with a validation feedback action, you could put an exclusive split with the same logic as the conditional visiblity on your tab before the validation feedback action, to only trigger the validation when the tab is visible.
If I place the fields in the hidden tab inside a container and also set the containers conditional visibility then the feedback validation isn't triggered for those fields.
Interesting to note: I asked Mendix if this was expected behaviour from a tabcontainer... their reply was: "we concider this a feature request"
How are you validating these fields? are you marking the fields as required? If so, have you tried adding the validation on the entity level? This will only trigger when you commit, so if there as an object in the hidden tab that isnt being comitted the validation won't run.
Another alternative is to use a microflow to validate. This way you can control the conditions on which fields will be validated. So instead of a standard save button, use a microflow button that will validate, commit the action, and then close the page.