I’ve been a barefoot trimmer for about 12 years, and am almost certain that what you’re seeing is nothing more than the periople swelling from the wet conditions. In horses the periople is a thin layer of soft horn somewhat like the cuticle of our fingernails, and is responsible for helping to regulate moisture in the hoof. It normally extends a short distance down the hoof wall all the way along the coronet (hairline) as you describe. If you see this thin soft horn further down the wall (say about a third to halfway) then it’s been irritated for some reason. What you describe would be of no concern to me, and I’d rather see more moisture available for hooves than too little. Also continual wet/dry cycles can be hard on hoof quality, much like what causes chapped hands.