Running LexForge 47.1.3 (1.20.1) with the following mods (all of which are Blueprint-based):

Autumnity [5.0.2]
Berry Good [7.0.0]
Blueprint [7.1.1]
Buzzier Bees [6.0.1]
Caverns & Chasms [2.0.0]
Clayworks [3.0.3]
Environmental [4.0.1]
Gallery [1.0.2]
Neapolitan [5.0.0]
The Endergetic Expansion [5.0.1]
Upgrade Aquatic [6.0.2]
Woodworks [3.0.2]

I observed inconsistent modded biome slices (they seem to get shifted randomly) when generating a world on the same seed (7616405129049401866) multiple times. I verified in the logs that Blueprint loaded the same number of chunk generator modifier groups each time.

Other potentially relevant information:
Windows 11 23H2, Adoptium jdk-21.0.5.11

I took additional steps to test on lexforge 47.3.0, neoforge 47.1.106, and the latest openjdk build, and can confirm that the issue still occurs in all cases.

Okay, after much testing, I finally got the same results. It looks like this is a rare vanilla bug, which is why nobody noticed and might even explain some of vanilla's seed inconsistencies.

What's happening is there is a race condition when Vanilla loads its data pack registries, rarely causing registry entries to be in random order. The slice selector assumes this order is always the same, but this can rarely change when CPU thread speed is inconsistent enough.

Unfortunately, this issue cannot be fixed without breaking existing worlds, so I will close this and wait for the next Minecraft version to fix it. The significant effects of this issue are very rare and have gone unnoticed for quite some time.