Pasty butt is caused by stress which includes too hot, too cold, to much handling, stress from shipping....etc
Chicks hatched under a broody never get pasty butt I notice.
These chicks are 1 or 2 weeks old.
I would be very supprised if they are eating the bedding.
I only keep my new born chicks on paper towel for three days. By then they know what is food and what is not.
They get large pine shavings after that, not saw dust.
Glad you are adding the electrolyte...
More questions:
What are you using for brooder and heat source?
Feeding what?
How's the temp in the brooder?
What are the symptoms before passing?
Photo of your setup would be nice.
Sorry this is happening!