Cactus doesn't destroy items when they are on top of it

I have recently built a cactus farm on Minecraft Bedrock Edition, and sometimes the cactus blocks pop on top of the cactus, but they don't disappear when they are on the cactus, so the only way to make them disappear is to wait for them to despawn after 5 minutes. Has anyone experienced this bug either if you have built the cactus farm or not? Thanks!