Missing TCA labels in inline tt_content relation - typo3

Question

We are using the Mask extension for some of our content Elements. (It basically adds dynamic CTypes and additional columns in the tt_content).
Next we have our custom extension whose entities should have a relation to an instance of a Mask module. Which works. The only problem is, that the labels of the inline element aren't showing up.
If we add the same Mask module as normal page content, the labels are showing up just fine.
Domain Model of our ext:
/**
* Returns the intro
*
* #return \TYPO3\CMS\Extbase\Persistence\ObjectStorage<\MASK\Mask\Domain\Model\Content> $intro
*/
public function getIntro()
{
return $this->intro;
}
TCA
'intro' => [
'exclude' => 0,
'label' => 'Intro zentriert',
'config' => [
'type' => 'inline',
'foreign_table' => 'tt_content',
'foreign_field' => 'tx_zeppelinregions_content_elements',
'foreign_label' => 'tx_mask_teasergalleryheadline',
'foreign_table_field' => 'tx_zeppelinregions_content_elements_type',
'maxitems' => 1,
'foreign_match_fields' => [
'ctype' => 'mask_introzentriert',
],
'overrideChildTca' => [
'columns' => [
'CType' => [
'config' => [
'default' => 'mask_introzentriert',
'readOnly' => 1,
],
],
],
],
]
],
Missing labels in inline element:
(Note: the default tt_content field labels are showing)
Labels showing when adding an element of the same type as normal page content: