No problem at all, apologies for letting this slip through the cracks. Things have been quite busy lately! This is actually a strange scenario; this endpoint's behavior is inconsistent with the tool, but the behavior (i.e. default Lifecycle Stage) is actually something that the team is reevaluating for the whole product. Therefore, this isn't something that the team is going to fix right now, since it's possible it might be part of a bigger change in the near future.
That said, I can understand how this is causing problems for you. I would strongly recommend setting the Lifecycle Stage property explicitly when creating contacts from here on out. If you wanted 'Subscriber' to remain the 'default' for contacts created via the bulk contacts API, you can simply include the Lifecycle Stage property for every contact you create.
Apologies for the delay here, let me know if you have any other questions!