Minecraft Bedrock has a different way of rendering X-shaped blocks including kelp. In this case, only the plant has a symmetry unlike the top part of the kelp. This is due to the flipbook_textures.json file which displays the textures in the wrong order. They should all start with 0 in this case it is not the case (proof if below, without modification.)

Below you will find the fixed flipbook_textures.json and all the correct texture of kelp taken from Java.