Put the summary of the bug you're having here

I tried to make one of those automatic cactus farms in MCPE with the introduction of water buckets I expected he cactus to pop out when it grows beside a block but didn't.

The cactus grows beside the block until I give it a block update which made it pop off.

Place a cactus with a block diagonally above it. Wait for it to grow and you will see that it does ot pop off until you update it.