TYPO3 Core

I have the following situation:

I have multiple check boxes which are required.
Like this:

 <f:for each="{organisations1}" as="organisation1">
   <label for="checkbox-{organisation1.uid}" class="menu-checkbox">
     <f:form.checkbox id="checkbox-{organisation1.uid}" property="organisation1" value="{organisation1.uid}" />
     {organisation1.title}
   </label>
 </f:for>

Everything works fine until i get a validation error. After that the check boxes are no longer correct:

Should be:

tx_ndecompanies_companies[company][organisation1][]

But is:

tx_ndecompanies_companies[company][organisation1]

As you see after a validation error the form is missing the [].
So the user is no longer able to send the form.

To fix that i use:

<f:form.checkbox id="checkbox-{organisation1.uid}" property="organisation1" value="{organisation1.uid}" multiple="1" />

So with multiple=1 it is working.

But for me it is a bug, that it is working without multiple=1 when there is no validation error and not when there is a validation error.