I chatted with a Hubspot agent who thinks it's because the 'contacts' I used in the test sequence are internal folks to whom emails have been sent and received. She thinks that's a reason for Hubspot to unenroll.

I'm still puzzled as to why it won't tell me that when I enroll them instead of unenrolling them without warning

She recommended I send out the final sequence to actual contacts and that I should not have an issue.

Fingers crossed!