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.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-88643 in the post.