Problem on forms fields evaluation in tabs designed from Nuxeo Studio

Fom Nuxeo Studio, I configure a tab the following way:

• One row only
• I first drop a header widget
• I then drop the tasks widget
• On the first widget, in the advanced configuration, I write an EL expression to hide my header widget when the lifecycle of the document is XXX
• I try my configuration on Nuxeo
• As soon as my document is in state XXX, the form bound to the current task of the document is "unactivated": it says the buttons are not included in a form. See attached screenshot.

It looks like the "hidden" value was evaluated for the wrapping <form> component too.