Hi Johannes,
It's the behavior of the WordPress core when you are in the backend, click Add new post, leave all blank (title, content ...) and just click Publish, it will save a new post with the title (no title).
So when you are in the frontend (not logged in), the extension MB Frontend Submission does the same thing, leave all fields blank and click Submit, a new post is created with the title (No title).
You can make a/some fields required when submitting the post by adding the setting required => true to the field or use the Validation.
https://docs.metabox.io/extensions/mb-frontend-submission/#reorder-post-fields
$meta_boxes[] = [
'title' => 'Bill Submit',
'id' => 'bill',
'fields' => [
[
'name' => 'Submission Date',
'id' => 'submission_date',
'type' => 'date',
],
[
'name' => 'Title',
'id' => 'post_title', // THIS
'required' => true,
],
[
'name' => 'Type',
'id' => 'type',
'type' => 'select',
'options' => [
'docs' => 'Document',
'receipt' => 'Receipt',
],
],
[
'name' => 'Description',
'type' => 'textarea',
'id' => 'post_content', // THIS
],
[
'name' => 'Thumbnail',
'type' => 'single_image',
'id' => '_thumbnail_id', // THIS
],
],
]
.
