Just to share in case it helps someone:
My mother has very advanced disease and her tongue doesn’t move well; she has difficulty swallowing and her mouth is usually open, so she mouth-breathes all day. She is immobile and nonverbal and stays in a care facility. On her last trip to the dentist (a caregiver had brushed her teeth beforehand) the hygienist showed me that a dry membrane-like coating had formed on the roof of her mouth from whatever food/drink hadn’t been able to be cleared by her tongue in the process of swallowing. The residue dries with her mouth breathing and adheres. I check her palate regularly now with my mobile phone’s flashlight and then remove any coating with a wet toothbrush—sometimes there is something stuck to her tongue as well. It appears to happen a few times a week, and giving her a thickened drink or even using a Water-Pik isn’t enough to dislodge it. I am in the process of telling the care staff about it.
So I would encourage people to check the palate carefully as part of the oral care routine. Otherwise who knows how long that material may stay there.