First Line Field empty when i use Multi Select in a drop Down Field

Hi
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A feature? A Bug?
When i set Multi Select in a drop Down Field to yes, the first line in the Dropdown on the Form Preview is empty.
Its correct when i set to "No"
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Thank you and best regards
Christoph

Ok
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This "effect" is only seen in the preview mode.
When i publish the form i can see the first lien also in multiselect mode.
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Thank you
Christoph