The bug

The way biomes are placed in the Nether, as well as the fundamental shape of terrain, differs in Java Edition and Bedrock Edition. As 1.16 introduced nether biomes, and 1.18 aimed to have the world generate the same on both editions, and both of these releases are after 1.15, this is a valid parity request. (The terrain shape element may not be valid as it has remained largely the same since Alpha v1.2.0, but is probably worth looking into nonetheless alongside this since the biome aspect certainly is valid.)

How to reproduce

The same seed must be used on both editions. 1 is used on Java Edition, and ddnqavbk on Bedrock Edition as it becomes 1 (single digits still cannot be input directly due to MCPE-44604).

1. In Java Edition, /execute in the_nether run tp 0 100 0
2. Note the biomes and terrain in this general region
3. In Bedrock Edition, teleport to (0,0) and place a nether portal
4. Note the biomes and terrain in this general region - there is a good chance they will not match up

For seed 1, Java Edition yielded nether wastes, whereas Bedrock Edition gave a crimson forest. Java Edition's terrain was wide open above a lava lake whereas Bedrock Edition's was in a more secluded cavern.

Expected results

The terrain and biomes would match up very well between editions.

Actual results

More often than not, the terrain and biomes are completely different.