I can't add a custom label on a multiple

In the forms to create and edit a job offer, I have to customise the label of the field for skills.

But I have a problem: it seems that when I add a widget on multiple-skills (like multiple-skills='sib-multiple-select') it is not taken into account.

Here is an example with and without a value for multiple-skills. Other labels are ok. https://jsbin.com/rujehubica/edit?html,output