Mining monster eggs in the extreme hills with a Silk Touch diamond pick axe causes them to break instantly but no block is dropped and no silverfish will spawn. If broken with anything else (hand, etc) the silverfish will spawn as intended. If it is of significance, I am playing on a server that has amplified terrain.
Thank you for your report!
However, this issue is a Duplicate of MC-48184.
Silverfish will not spawn if you mine Infested Blocks with Silk Touch. Also, I cannot reproduce Infested Blocks not dropping blocks.
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