Not a direct answer to the bug, but an alternative to making the exec_mode=blocking.
I hadn't experienced the issue in the last 7 years using exec_mode=normal, recently I've begun migrating to PHP 7.1 which is when I first noticed the bug and it occurred every request. During debugging sessions, the bug would only occur some of the times, I noticed when I was digging into the create method, the request would succeed.
I placed a one second sleep after the create and before my isDone loop and the request succeeds. I haven't looked into the cause of the bug itself, but I suspect at least for myself when running under PHP 5.3.8 the processing is that much slower than PHP 7 that it was enough of a delay to not trigger the bug. I would wager that the job in splunk hasn't been fully created by the time the script was looking for a status.