Between 1.15.2 and 1.16.1, portal creation/synchronization broke.

When first exiting the nether, a new portal will be generated if one does not exist in that area.
When returning to the nether, the correct portal will be located and used.
When re-using that same portal, it appears to (usually [1]) bring you back to the same portal.

If the overworld portal is moved just 2-3 blocks, the bug becomes apparent;
After moving the portal a few blocks and going to the nether, all is still fine.
When returning to the overworld, the player is dropped off at the original portal location with no portal being generated.
The player can then use their overworld portal again to get correctly to the nether.
Every return to the overworld will result in being returned to the phantom portal.

[1] - In some cases, a player will seem to be randomly sent to one of two phantom portal locations.

This has been reliably reproduced on MCv1.16.1 in multiple chunks, including chunks that are part of regions which were deleted and re-generated.