The doMobSpawning gamerule controls whether mobs spawn naturally based on the global spawning logic, but does not affect special spawn conditions like monster spawners, raids, etc.. Since trial spawners function in a somewhat similar fashion to normal monster spawners, it is supposed to be considered as a special spawning attempt, thus, being able to function regardless of the doMobSpawning gamerule setting, but it is not the case. The attached image where the gamerule is set to true shows that the trial spawner is active and can spawn mobs, whereas the other attached image shows the trial spawner being disabled as the gamerule is set to false.