You haven't mentioned if you've examined the feet of the limping chickens. It's possible to have multiple cases of bumblefoot when the coop lacks adequate bedding to cushion the dismount off the roosting perches. I would rule that out before going further. Also check the scales for lifting away from the shanks and the appearance of crusty dirty stuff under them indicating leg mites. This, along with bumblefoot can cause limping.
The coop floor is a mix of masonry sand and screenings... But you have a good point. With the adult hens, it would be possible that they could have hurt themselves jumping down from one of the high roosts. My highest roost is approximately 5 ft off the ground, but a handful of the hens and both of my adult roosters have taken to jumping up into the rafters of the coop and sleeping there instead. If one of them jumped from that height, it's at least a 6 - 7 ft drop. They don't HAVE to jump from that height though... The roosts are situated so they can easily jump up and down from one to another at different heights.
That being said, one of the young pullets that was limping is a buff orpington. I haven't seen ANY of my buff orpington pullets on any of the higher roosts. They usually get relegated to the lower roosts which are only 18" off the ground. I don't think I've ever seen one on any roost higher than 3 ft.
Long story about mites. I bought this house back in Feb 2020. The previous homeowner passed away in the middle of the sale. He had about 15 chickens which I adopted as part of the sale. When I looked the chickens over after taking possession of the house, it was clear that many of them had evidence of scaly leg mites. Over a period of a week, I treated every single on of them by coating their legs with vaseline and dusting down the birds with diatomaceous earth. I also sprayed down the entire coop they were using with miticide on multiple occasions. It seemed to clear up the issue. That was back in Feb/March 2020. In Oct 2020, all the chickens were moved into a brand new coop.
While some of the "adopted" chickens legs still look "thick," none of the chickens seem to exhibit any other signs of mites (no crustiness, itching, etc...). Two of my original chickens which I brought with me in the move have had the limping I described. Neither of these girls has ever had scaly leg mites. Their legs are still smooth as silk....and they have been limping here and there. One of them sleeps on a ladder roost (not jumping from big heights) and the other sleeps on the 5 ft high roost. Lastly, the buff orpington pullet that is currently limping is also not showing signs of leg mites.
As for bumblefoot, I know a couple of the chickens DID have signs of bumblefoot the last time I went through the coop and looked at everyone which was back in the fall. There were at least 4 or 5 cases...although only 1 was limping at that time. The rest just exhibited the brown scabs on their bottom pad but it didn't seem to bother them much.
Based upon your advice, I will go through the coop tomorrow night and assess everyone for both mites and bumblefoot.... Better to rule it in or out definitively.....