Structure bounding box shifts in pre-generated chunks after enabling Tectonic

[Kojjyan]

Bug: Structure bounding box changes in chunks generated before installing Tectonic.

Evidence:
Before Tectonic: Ocean Monument bounding box at correct Y level (MiniHUD mod marks the structure bounding box in light blue).

After Tectonic: Bounding box shifts down to Y:9–32 in same chunks. This causes all the guardians to spawn below the monument in a crevice in the light-blue area.

Expected behavior:
Pre-generated structures should retain original bounding box and location after enabling the mod.

Consequence:
Existing guardian farms based on original structure bounds break due to shifted spawn area.

Version:
tectonic-3.0.19-fabric-1.21.11.jar
fabric 1.21.11
Same thing happens whether or not I enable terralith.