It can cause bumble foot. I am just going off my experinces and others. But everyones experiences will not be the same. All I can say is your lucky you have not experienced bumble foot.
I assume your egg layers would have some substrate in their run. Because your free ranging chickens are free ranging they have less off a chance to step in their droppings, there for less chance of infection.
Is this meant to mean you let them free range in the run then? That's not free ranging. That's confined ranging. When I first got chickens I had a lot of rubble in the run. I have since removed it. I have not gotten another bumble foot case since then.
Chickens get bumble foot because they cut themselves. Are you saying that a chicken who is on sand all day - for an example, will get bumble foot? If a chicken doesn't have an injury no infection can enter their feet. Some people have the luxury of letting your chickens free range - such as you...
If you have a rocks/stones in the run, they can cut themselves and this may cause bumble foot. Check your chickens if they have it and then remove the rubble before putting down the deep litter.