Consider the following relationship

Tag <——- M:M ———-> Question

And the two entities are defined as follows in Kotlin, in database the two entities are linked with a joint table tag_question

@Entity
@Table(name="tag")
data class Tag(
    override var id: Int,

    @Column(name="name")
    var name: String,

    @Transient
    @ManyToMany
    @JoinTable(name = "tag_question",
        joinColumns = [
            JoinColumn(name="tag_id", nullable = false)
        ],
        inverseJoinColumns = [
            JoinColumn(name="question_id", nullable = false)
        ]
    )
    var questions: List<Question>? = mutableListOf(),

    override var createdAt: Timestamp,
    override var updatedAt: Timestamp
):Base() {
    constructor():this(0,"",
        mutableListOf(),
        Timestamp(LocalDateTime.now().atZone(ZoneId.systemDefault()).toInstant().toEpochMilli()) , Timestamp(LocalDateTime.now().atZone(ZoneId.systemDefault()).toInstant().toEpochMilli()))

    fun toDTO():TagDTO {
        return this.let {
            TagDTO(it.id, it.name,
//                it.questions?.map { que -> que.toMemberDTO() },
                it.createdAt, it.updatedAt)
        }

    }

    fun toMemberDTO(): TagMemberDTO {
        return this.let {
            TagMemberDTO(it.id, it.name, it.createdAt, it.updatedAt)
        }
    }
}

@Entity
@Table(name="question")
data class Question(
    override var id: Int,

    @Column(name="content")
    var content: String,

    @Column(name="summary")
    var summary: String,

    @OneToMany(cascade = [CascadeType.ALL], mappedBy = "questions", fetch = FetchType.EAGER)
    var answers: List<Answer>,

    @ManyToMany(mappedBy = "questions", targetEntity = Tag::class, fetch = FetchType.LAZY)
    var tags: Set<Tag> = mutableSetOf(),

    @ManyToMany(mappedBy = "questions", targetEntity = com.questionset.model.Collection::class, fetch = FetchType.EAGER)
    var collections: List<com.questionset.model.Collection>,


    @ManyToOne(cascade = [(CascadeType.DETACH)], fetch = FetchType.EAGER)
    @JoinColumn(name="course_id", nullable = false)
    var status: QuestionStatus,

    override var createdAt: Timestamp,
    override var updatedAt: Timestamp
):Base() {
    constructor():this(0,"","", mutableListOf(), mutableSetOf(), mutableListOf(), QuestionStatus(), Timestamp(LocalDateTime.now().atZone(ZoneId.systemDefault()).toInstant().toEpochMilli()) , Timestamp(LocalDateTime.now().atZone(ZoneId.systemDefault()).toInstant().toEpochMilli()))

...

I am trying to fetch all tags with its questions in a single repository query

    @Query("SELECT t FROM Tag AS t JOIN FETCH t.questions")
    override fun findAll(): MutableList<Tag>

However I receive following errors

java.lang.ClassCastException: class org.hibernate.metamodel.mapping.internal.BasicAttributeMapping cannot be cast to class org.hibernate.sql.ast.tree.from.TableGroupJoinProducer (org.hibernate.metamodel.mapping.internal.BasicAttributeMapping and org.hibernate.sql.ast.tree.from.TableGroupJoinProducer are in unnamed module of loader 'app')

Possible errors that I have double checked:

1. I am NOT missing @Entity annotation for the two classes.
2. I am using FetchType.LAZY for Question and FetchType.EAGER for Tag

How to make hibernate treat questions property like relationship, not basic attribute?