terrain_matching projection generate on top of vegetation

Summary: Structure pieces with terrain_matching projection generate on top of vegetation when chunks finish generating after being unloaded mid-structure

Game Version: 1.21.1 (reproduced on pure vanilla with no mods)

Category: World Generation

Description:

When a jigsaw structure that uses terrain_matching projection begins generating across multiple chunks, and the player moves away before all chunks have finished generating, the remaining chunks unload mid-structure. When the player later returns and those chunks finish generating, the terrain_matching structure pieces (such as village roads) are placed on top of vegetation (tall grass, flowers, ferns, etc.) instead of replacing it and snapping to the solid ground beneath.

This is incorrect behavior. In a normal single-session generation flow where all chunks generate without interruption, terrain_matching pieces correctly place at solid ground level, replacing any vegetation in their columns. The bug only manifests when chunk generation is interrupted by the player moving away or leaving the game, causing the remaining chunks to finish generating at a later point after the vegetation-inclusive heightmap has already been populated.

A workaround exists: deleting the affected chunks using a third-party chunk editor forces them to regenerate cleanly, which produces correct placement. This confirms the correct placement is calculable — the bug is in the heightmap being read at the wrong time during deferred generation, not in the structure data itself.

Important Note on Mod Involvement:

This bug was investigated extensively before being confirmed as vanilla. Testing was done progressively stripping back mods one by one, including removing Lithostitched (which overrides vanilla jigsaw generation as of its 1.3.5 changelog), Distant Horizons, C2ME, and all other mods entirely. The bug persisted at every stage of this process. Final confirmation was done on a completely unmodified vanilla 1.21.1 installation with no mods, no Fabric, and no Fabric API — just the vanilla jar. The bug reproduces reliably in this environment. This is not a mod conflict.

Steps to Reproduce:

1. Launch pure vanilla Minecraft 1.21.1 with no mods installed

2. Create a new world or use an existing one

3. Locate a village or any jigsaw structure that uses terrain_matching projection pieces such as roads

4. Approach the structure close enough that generation begins, but do not fully reveal all chunks of the structure — the render distance edge should visibly cut through the middle of the structure

5. Walk away from the structure or save and quit the world before the remaining chunks have loaded and generated

6. Return to the structure in the same session or a new session and allow the remaining chunks to finish generating

7. Observe the newly generated terrain_matching pieces

Expected Result: terrain_matching structure pieces replace vegetation and snap flush to solid ground, consistent with pieces generated in a single uninterrupted session.

Actual Result: terrain_matching structure pieces are placed on top of vegetation — tall grass, flowers, ferns and similar — as if the heightmap query returned the top of the vegetation layer rather than the solid ground beneath it. The lower half of tall grass remains visible underneath the structure piece.

Workaround: Deleting the affected chunks using a third-party chunk editor such as NBTExplorer forces them to regenerate cleanly, correcting the placement. This workaround confirms the bug is in the timing of the heightmap query during deferred generation, not in the stored structure or chunk data.

Additional Notes:

• This bug has no existing report on this tracker despite likely being present across multiple versions

• The _WG heightmap variants (e.g. WORLD_SURFACE_WG) used during worldgen are transient and are explicitly deleted after carvers run. When chunks finish generating their structure pieces after a session boundary or chunk reload, these heightmaps are no longer available. This may cause a fallback to a non-_WG heightmap type that includes vegetation in its calculation, producing the observed incorrect placement

• Affects all jigsaw structures using terrain_matching projection, not just villages

• The bug is consistent and reproducible — it is not intermittent

I used claude to generate this detailed report. But its all accurate. I know this is an old version (1.21.1) But ive found zero related issues so I think it was best for me to report. Again, I use alot of mods, but ive stripped down all of them to find the culprit and even in vanilla it still caused this issue.