The Bug:

When water flows against a cactus, it doesn't break. The water simply goes around it. 

Other blocks like (tall) flowers, (tall) grass, dead bush etc. will break upon contact with flowing water.

This seems to be an intended feature, but cactus will instantly break when water is placed on top of it while other alike blocks, like the ones i mentioned above, don't instantly break when water is placed on top of it. (There is a small delay of maybe a few ticks)

Expected result:

Water placed on top of a cactus shouldn't instantly break, because you can place blocks on top of a cactus. 

There should be a slight delay like when water is being place on top of flowers, grass, etc.