Dynamically-spawned instances of a Blueprint class go through an optimized property initialization pass (see UBlueprintGeneratedClass::BuildCustomPropertyListForPostConstruction() and InitPropertiesFromCustomList()). This code path does not properly account for native struct-typed values that either (a) specify their own Identical()/Copy() functions, or (b) do not declare any UPROPERTY() fields. FDateTime is one example of (b) that we currently have in engine code, which also includes a property type customization that allows the default value to be modified. As a result, the struct value in the spawned instance will not currently receive the modified value from the Blueprint CDO.
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There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-88643 in the post.