Corgis sometimes roll in their own waste and other things.
For hunting (birds, deer, and other things), dogs in general have to cover up their scent by rolling in something a lot more powerful or worse smelling then theirs. Corgis, like most dog breeds, have this ingrained through evolution.
This instinct is hard to correct to where your corgi will not roll in something that smells. But it may take some time till your corgi has learned not to roll in something that smells.
Two things to help your corgi learn faster is to become alpha leader, and make sure your “NO” is given as a command during or immediately before the time your corgi starts to roll in something he shouldn’t.