Senna's passive reads, "Small minions and monsters have a 20% chance to spawn Mist Wraiths, reduced to 5.5% chance on those that Senna kills."
When Senna executes a non-cannon minion via Spoils of War (the gold generation passive on Steel Shoulderguards and Relic Shield), my testing indicates souls have a 5.5% chance to drop. Is this correct?
Is this the intended behavior, or a design oversight?
Why are you even questioning this?
Senna killed it. It gets a lower mist drop rate. Just because she shares the gold with her ally doesn't mean she didn't kill it.
I'm confused what you're asking. Senna executing a minion with a Relic shield is her killing it. So yeah.... it would have the 5.5% rate (soon to be only 1.86%)
It's not like as a Relic shield build you kill a "Lot" of minions.
Yes it's working as intended 
W with a support who takes the balloon also won't drop a bubble.
So the chance is reduced to 5.5%, and your experimentation result is 5.5%?
