[SalesForce] Marketing Cloud Journey Builder Entry Event not evaluating any Contacts for entry

I'm trying to create a new Shared Entry Event for my Interactions in Marketing Cloud Journey Builder and not having any luck getting them to work. I've scoured the documentation, and related Q&As here, but to no avail. Here's where I'm currently at:

1. I've created a new Sendable Data Extension that contains SubscriberKey and EmailAddress attributes along with my own attributes. SubscriberKey is the primary key.

2. I've linked my DE to the Contacts data schema as a Population in Contact Builder, from ContactKey <=> Subscriber Key. When I view the DE details, it DOES show up with Use as root checked (even though this checkbox is supposed to be deprecated).

3. In Contacts Configuration, I've added my DE and its EmailAddress attribute to the Email Address channel order, in the 1st position.

4. In Journey Builder, I've created a new shared Contact Event, with my DE as the Entry Source, NO filters (so all new records should enter), and turned it on to Test mode.

5. I then hit the API, first /contacts/v1/contacts so that I can create the Contact, with valid Email Address and Email Demographics, then with the same contact/subscriber key, I hit /interaction/v1/events with the rest of my DE data.

6. I get success responses from the API calls, and a new eventInstanceId back (where can I look this up??!), and if I check All Contacts in Contact Builder, I can see the valid Email contact record AND the associated data in my DE.

7. However, when I go back to Journey Builder Event Administration and view Event Performance, it shows 0 contacts evaluated for entry.

I've also tried manually creating/importing contact records, then firing the event with an Automation, but it also still shows 0 contacts evaluated for entry afterwards (and I really want to just use the API for my integration).

Can anyone see something I may have missed in my steps above? Happy to provide additional screenshots/details as needed.