Isolated to Minecraft 1.12.2, Forge: 14.23.1.2582, no other mods in build.

Mod appears to be preventing Silverfish from spawning naturally or forcibly (egg spawner, monster spawner blocks or creative summon command), even when no settings are changed or even setting true to silverfish directly.

Duplicate of #18

This is now fully working with version 1.12.2-7.0.0 and Forge 14.23.2.2646 or higher.
Silverfish will both spawn and be handled by the mod as expected.