Something isn't right, need to figure out WHY this happens. Are you sure there are no mites? They're very tiny, almost impossible to see, often it just looks dirty at the base of a clumpy feather, easier to see on a Buff feather color. The few times a couple of my chickens looked like they had something going on by vent, there were tiny mites. I couldn't tell by looking at the bird on my lap, I had to pull a little dirty feather out & hold it under a magnifying glass to see the miniscule things barely moving! I caught very early. If left alone they start chewing the poor chicken skin, making a scabby mess, and all you see is dry skin or scabby, flakey skin, the darn mites hide under the scabs, so they're extremely difficult to see. I'm far sighted so I need a bright light & magnifying glass to even see the little hideous parasites.
Whatever it is, you need to find out why.
Ok I just had time to watch your video, they look happy & healthy, laying eggs daily is good.
I have 1 feather plucker in my flock of 38. To this day, she still plucks her Roo's tail feathers, but he is so sweet he let's her! Why she does this is a mystery to me. Some suggest it is their way of getting more protein, but I don't know, I supply a higher protein feed during molt & do offer a hand full of mealworms (1 handful per 4 hens) a few times a week year round. My hen was plucked totally bald as a youngster, traumatized, so when I got her, every other chicken was chasing her, she was literally bald. I went there for 3 hens, ended up rescuing a dozen plus the baby Roo, just to get them out of that nasty place. All Ameracaunas or EEs. When I brought her home, I set her up in her own cage to calm down & let feathers grow. Once she had feathers, I introduced her to a few hens at a time while free ranging, and they did accept her, but she was mean to them, ripping out feathers at 1st, I guess it was payback LOL. She did finally become calm & not as grumpy, but she still pecks out Baby Roo's tail, he is an adult now, but younger than her.
The preen or uropygial gland is not at the vent btw, it is up on top of the back, before where tail feathers begin, if you get a hen to sit on your lap and look you can see, it almost resembles a human wart or oily pimple Lol.
I don't know what to tell ya...if it isn't mites, not sure, I've never encountered bald butt syndrome here.
They do like dust baths and have a fav spot, I call it the dust bowl, because it becomes like a big bowl. I add some mixture & they love it...in a 5 gal bucket mix dry soil, some playsand & DE to the bowl. This has helped prevent mites.
Sorry I can't be of more help.
(My chickens dust bathing in their dustbowl.)