Jackson - @JsonTypeInfo property is being mapped as null?

Finally, I've found the solution in the API Doc

Note on visibility of type identifier: by default, deserialization (use during reading of JSON) of type identifier is completely handled by Jackson, and is not passed to deserializers. However, if so desired, it is possible to define property visible = true in which case property will be passed as-is to deserializers (and set via setter or field) on deserialization.

So the solution was simply adding the 'visible' attribute as follows

@JsonTypeInfo(  
    use = JsonTypeInfo.Id.NAME,  
    include = JsonTypeInfo.As.PROPERTY,  
    property = "exercise_type",
    visible = true)  
@JsonSubTypes({  
    @Type(value = ExerciseChoiceResponseDTO.class, name = "CHOICE"),  
    @Type(value = ExerciseQuestionResponseDTO.class, name = "QUESTION")})  
public abstract class ExerciseMixIn  
{}  

Hope this helps someone else.

As per @jscherman answer by setting, 'visible' true in JsonTypeInfo will help in accessing exercise_type as a field.

If you use the same class to serialize also then resulting JSON will have exercise_type appear twice. So it's better to also update include to JsonTypeInfo.As.EXISTING_PROPERTY

And it's also worth looking at all other options for include.