The Nether Portal is teleporting to the Nether using the coordinates (approximately) when the player teleports while sleeping in a Bed.

I Attached 4 .png to show what happens:
The portal is linked correctly to the Nether (as shown in the images Portal Original Location and Nether Original Location). But if the player is teleported while sleeping in a bed, a new portal is created, but the Nether coordinates are the same of the Overworld player's coordinate.

When the player comes back from this new portal, it's teleported to the location corresponding to the new Nether Portal (as shown in the images Nether After BedPortal and Overworld BedPortal).