Pretty sure it has to do with how he "went back in time", since it happened from grabbing the demon lord's gem, so there is a chance he himself either turned into a half demon or that a part of the demon lord resides inside him(since the demon lord speaks in plural, even though that might also be because "they" don't have a gender but w/e)
Or it could be that part of his soul is standing in a different time space... After all, even the Dragon King and the Magic King said that they see a blur in his soul.
But it would also makes sense what they said, about a part of the Demon Lord residing with him, after all, it was the Demon Lord's magic that was used to cast the Time Spell... Plus, if you read between the lines in the earlier chapters, I also had a little suspicion that MC felt like something is just a little fishy, which made me think about the Time Spell and the Demon Lord.
Anyone wonder or understand why the Demon Lord asked why our MC was standing over there
You know before the unwanted guests interrupted