This paper presents DITTO-Mirror, a Mixed Reality (MR) system that explores how users construct, maintain, and question fictional logic during interaction. In DITTO-Mirror, a virtual object appears to be reflected in a real mirror, despite the absence of any physical reflection. This illusion is created using two identical virtual objects: one is directly manipulated by the user, while the other is placed in a mirror-symmetric position relative to the real mirror, creating the appearance of a reflection. Users interacting with this system readily accept the mirrored object as part of the real world, exhibiting behaviors consistent with their belief in its physicality. However, when a second mirror is placed—one not recognized by the system and therefore not displaying the mirror virtual object—users experience discomfort and confusion. This reaction highlights a disruption not in visual realism, but in the fictional coherence they had come to internalize. These moments reveal that belief in MR is actively constructed and can be strategically challenged to provoke reflection and curiosity. We argue that MR can serve as a medium of designed fictionality, where belief and disbelief are not passive states but dynamic, interactive processes.