Calendar opens in the background

Hello, I noticed the following: If I use an overlaying section with a button and this contains a form with a calendar function, the window of the small calendar opens in the background and is probably not visible to the surfer. You can test this on https://bueromanagement-nagelreiter.at/ using the button: “Schreiben Sie uns” above.
If you could help me here that would be great.

Thank you and best regards

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