ClassCastException: Family$1 cannot be cast to UndergroundRootsFamily on server startup (NeoForge 1.21.1, dtbop 3.4.0)

[Smiley-droid]

Describe the bug

Dedicated server fails to start during registry baking. The crash happens
in BasicRootsBlock.getFamily, where a root block resolves to the generic
Family$1 placeholder instead of UndergroundRootsFamily.

This looks similar in nature to #1030 (dtbop:palm not resolving to
PalmFamily), but for underground roots instead of palms.

Versions

• Minecraft: 1.21.1
• Loader: NeoForge 21.1.234
• Dynamic Trees: dynamictrees-neoforge-1.21.1-1.7.2
• Dynamic Trees - Biomes O' Plenty: dtbop-3.4.0 (NeoForge)
• Biomes O' Plenty: biomesoplenty-neoforge-1.21.1-21.1.0.13
• TerraBlender: terrablender-neoforge-1.21.1-4.1.0.8
• Java: 25.0.3 (Oracle)

Steps to reproduce

1. Install dynamictrees 1.7.2 + dtbop 3.4.0 alongside Biomes O' Plenty and
   TerraBlender on a NeoForge 1.21.1 server (large modpack, ~70 mods total).
2. Start the server.
3. Server crashes during mod/registry loading, before world generation.

Crash report

crash-2026-06-21_17.29.36-fml.txt

2026-06-21-1.log.gz