TYPO3 Core

Hi everyone,

I have created an plugin with an flexform which has an field with type check.
Here im using the flexform to add 3 items and a userFunction to add 3 checkboxes which are shown correctly in the plugin.
However I noticed that when I check certain boxes and save the plugin, they are not saved.
Attached screenshot with end result from userfunction.

I tryed to add only items from the flexform these can be saved the correct way so it must be an problem in the userFunction.

<items>
<numIndex index="0">
<numIndex index="0">Test</numIndex>
<numIndex index="1"></numIndex>
</numIndex>
<numIndex index="1">
<numIndex index="0">Test2</numIndex>
<numIndex index="1"></numIndex>
</numIndex>
<numIndex index="2">
<numIndex index="0">Test3</numIndex>
<numIndex index="1"></numIndex>
</numIndex>
</items>

Has anybody else experienced this problem aswell in TYPO3 7.6.2? or did I make an mistake in my userFunction?