Breaking monster egg blocks in survival does not release silverfish

What I did:
Broke a monster egg block (found in a stronghold) with a Silk Touch Pickaxe

What I expected to happen:
The block to dissolve and silverfish to start attacking me

What actually happened:
The block dropped as a normal (mossy/cracked)Stone Brick, and no silverfish were released

Notes (For possible additional help): This only happened with a silk touch pickaxe, breaking the blocks any other way did release the silverfish. The world is a Customized World type, locked in hard mode, with natural regeneration turned off and keep Inventory turned on. These certain monster eggs were in a stronghold, the southernmost one. It happened with all three of the stone brick variants (Cracked, Mossy, and Normal).