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  
{}