use CascadeType.PERSIST

@ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.PERSIST)
@JoinTable(
    name = "ENTITY_BUTTONS",
    joinColumns = @JoinColumn(name = "ENTITY_ID"),
    inverseJoinColumns = @JoinColumn(name = "BUTTON_ID")
)
private List<Button2> entityButtons;