While bone mealing kelp in flowing water occasionally it will randomly stop as if there is no more water above for it to grow in making it difficult to create large bubble columns without have to place every water sorce block
https://photos.app.goo.gl/1jN2iSirkFw1cSLg9
Video of this occurring since it keeps failing to upload as an attachment
This issue is being tracked at MCPE-117366.