The block entity ID of spawners is mob_spawner, whereas the block/item ID is spawner (since 1.13-pre5).
The block/item got renamed to "Spawner" along with the ID in 1.13 (from mob_spawner), but the block entity ID was left behind in this change.

The Block entity ID is expected to match the (basic) block ID like every other block.

NOTE: the change from "Mob Spawner" to "Spawner" was made because the block is not limited to mobs, nor monsters; so the expected ID (and name) is "spawner".