When a Silverfish is affected by the Oozing effect and dies while on fire, it fails to produce a Slime (either as an entity or item drop) as expected. This issue occurs consistently under these specific conditions.
MCPE-235733
Silverfish with Oozing Effect Does Not Drop Slime When Killed by Fire
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I've found that many mobs like chickens and pigs won't spawn slimes when killed in this way either.