From my testing, it seems that the range of the magic mirror is multiplied by 8 in the overworld when your spawn is set in the Nether. I set my spawnpoint at a respawn anchor next to a nether portal, and I went to the overworld coordinates that matched up with the respawn anchor's. Up until you go past the relic's range * 8, you're able to use the mirror and it takes you back to the respawn anchor. If this is intentional behavior, I believe that this is too overpowered and it makes the mirror's range too big, as you could just have a respawn anchor right next to your house through a portal.

Also, using the mirror to teleport between dimensions seems to visually reset your experience bar, but gaining experience or rejoining the world fixes it. I'm not sure if this is a problem with the mod or with Minecraft.