Both the darkstride set and prisoner's rags set reduce sprint cost by 50%, so when you wear both it makes sprint free.
This works, but only until you weapon swap. After that, sprinting costs stamina again(even if you swap back to the bar where it was free before). This bug persists until you manually unequip one of the set items and re-equip it. The sprinting is free again, until you weapon swap. Video:
https://youtu.be/x3zEEVcghL4
First i sprint - its free. The i swap weapons back and forth - sprinting not free. Then i unequip and reequip shield - sprinting is free again.