We have one that has just been backed who does this.
We tried a variety of bits that we have in our stables. No change. He fiddled about, chomped, chewed, went around with his mouth open, like a ruddy goldfish. We even tried bitless and he bombed about like a goldfish!
We got the vet in to check his teeth as the horse dentist was not coming out (Covid and we are on an island with no horse dentist). Vet (who has a horse herself) put in gag, rummaged about and said everything was fine. No wolf teeth, no sharp bits, no problems. All perfect. Like I said, vet has a horse herself and has done Equine Science Masters Degree too.
My daughter (who likes research and science-based fact and has Equine Science Masters degree too) researched all types of bits and we got many in from the Bit Bank on approval to try. We found one that sort of was slightly better than the others but he wasn't "cured"..... Straight bar, titanium -
https://fagerbits.com/store/titanium-bits/sara-titanium-loose-rings
Today, after a ride out on Taktur and Klængur, we brought Efstur in to try out some bits Daisy had received from The Bit Bank. Efstur’s is a fiddler and chewer with his bit. The Bit Bank hav…
myshetland.co.uk
Equine dentist then announced he was finally coming to the island so we quickly booked an appointment. He examined horse, found mega-sharp bits and was shocked that vet had said everything was ok.
Equine dentist spent a lot of time on horse's mouth and now our horse is back in training and 100% different. Better, stopped chewing, stopped fiddling about with his mouth. He is much more settled and, dare I say it, normal!
So, moral of this long and boring story - get a well-qualified equine dentist. Not someone who did a coffee-morning in it who owns a gag and a couple of rasps.