When silverfish spawned from the trial spawner, they can infesting the stone bricks below, making the spawner considered they are dead, although the mobs spawned from trial spawners are designed not to despawn, and the stone bricks below the spawner are only used for decoration. There is no point in trivial to defeat them with stones.

Seed: 4631698291201647797

Coordinate: 453 -32 -415

Steps to reproduce

1. Find a silverfish trial spawner via seed and coordinate above.
2. Surround the spawner by using any block
3. Set your game mode to survival
4. Switch back your game mode

→ Silverfish "despawn" due to it infected the stone brick below, then the spawner spawning the next mob.