Indeed the value of the widget can be set to non-date value only during initialization.
As I can confirm that this is not consistent behaviour, I forwarded it to our developer's team, to check which behaviour is more expected and make it the same for both scenarios.
For now, as a workaround, I can suggest to destroy the widget and recreate again with a non-date value. As this is not a very complex widget, this should not cause big performance issues depending on how many datePickers are affected:
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