The "minecraft:spawn_mobs" game rule affects Trial Spawners

Trial Spawners are unable to spawn mobs if the minecraft:spawn_mobs game rule is set to “false”.

This is inconsistent with Monster Spawners, which are instead controlled by the minecraft:spawner_blocks_workgame rule.

The problem can be found in net.minecraft.world.level.block.entity.trialspawner.TrialSpawner#canSpawnInLevel:

public boolean canSpawnInLevel(ServerLevel serverLevel) {
    if (this.overridePeacefulAndMobSpawnRule) {
        return true;
    }
    if (serverLevel.getDifficulty() == Difficulty.PEACEFUL) {
        return false;
    }
    return serverLevel.getGameRules().getBoolean(GameRules.RULE_DOMOBSPAWNING);
}