Kelp does not create source blocks in downward flowing water

Trying to create a bubble column. Created a hollow column about 25 blocks high. removed two blocks and put signs here to prevent water from escaping. in the column, dug down one block, placed sand. went to the top of the column and emptied a bucket of water into the column. went back to the bottom and noted downward flowing water. planted kelp in the sand, all the way to the top. returned to the bottom expected to see source blocks, I didn't. I saw water STILL flowing downward despite kelp STILL being planted in those blocks.  Changed sand to soul sand, no change. Dirt, no change. completely sealed the column, no change. waited for an hour, no change. According to the wiki making a bubble elevator is perfectly legit, and under the kelp heading it specifically says that kelp will turn downward flowing water to source blocks. Yet, in the realm I am in, it doesn't..