Per the Note section here,
SplitOn is not synchronous. So behind scenes, it'll create a separate Logic Apps instance per iteration of the array, and as each iteration starts processing, they will immediately send a response with
202 success code which is probably what's starting your parent workflow too soon.
If that appears to be the problem, I'd recommend adjusting the workflow steps just a bit to incorporate the use of For Loop with the sequential looping turned on: "Foreach" loop: Sequential
If you don't care about the order of sequence, on the parent Logic Apps side, you can try turning on the Asynchronous Pattern setting: Asynchronous request-response behavior