Date field not valid if date is added dynamically

I have a dropdown select that gives you 3 date options. 2 Pre-calculated and another where you select your own date from a date picker.

The select updates the own with the first 2 preconfigured options:

When I do the form post I get an error with the first 2 options, even though they show the calendar input select as updated and the format is exactly the same:

Only after selecting own date on the picker the date is accepted. Not with a value update via the select.

Is this a bug or am I doing it wrong OR is there a better way?

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