I fight D1 Explore Mythic Bosses when I’m actively soul farming, mainly because swapping over to D12 to use up my stones once an hour feels like it breaks up the flow of soul farming. (Since it usually takes multiple D12 runs to drain it to a range when soul farming won’t hit 100 mythstones super quickly).
If I’m just soul farming until I hit over 100 mythstones one time, then I just swap over to D12.