Very closely relates to MC-250696, its Java Edition counterpart. Also very closely relates to MCPE-154333, and by extension of both, MC-250445.

The bug

Bedrock Edition 1.18 intends to make seeds 64-bit, since they were previously 32-bit. While this results in the Overworld having 2^64 different ways of generating, it seems as though the Nether was left out of this, as there are still only 2^32 possible generations.

How to reproduce

This can be easily seen by generating two worlds, one with a given seed and another with a nonzero integer multiple of 4294967296 added to that seed. I've specifically opted to use 10 and 4294967306, respectively.

Once these two seeds are generated, the Nether is to be entered and the player must go to the same coordinates in both worlds.

Expected results

Since these two seeds are completely different numbers, the Nether dimension shoukd generate completely different for these two worlds, especially since that's what the Overworld did.

Actual results

It did not, and as such we have two complete copies of the Nether in two completely different worlds.

How to fix

The Nether's generation should be changed such that all 64 bits are utilised.

This should preferably also be fixed for Java Edition at the same time (MC-250696), and in the same way as well, to achieve parity on that front (MC-244149).

Further notes

Once this is fixed, terrain and biome blending should be enabled for the Nether to get rid of any abrupt chunk borders.