How Exchange Online automatically cares for your public folder mailboxes (via AutoSplit)

Hello Eriq VanBibber sorry for taking some time to reply to you as I have to validate such information with our engineering team.

Answering your question in 2 parts -> consider the scenario that you have 200 PF MBXs where 100 PF MBX are hierarchy serving mailboxes while the other 100 are content based PF MBXs on such a scenario it's normal that PF MBX 101 should not have a complete hierarchy as its function is content based PF MBX not hierarchy serving, for more information on EXO limits please check the below article.

You are limited to 1,000 public folder mailboxes, and the maximum total size of all public folder mailboxes is 100 TB. Hierarchy Serving mailboxes are limited to 100 public folder mailboxes. Exchange Online limits - Service Descriptions | Microsoft Learn

Consider another scenario that you had 40 PF MBXs where some of these PF MBXs have IsExcludedFromServingHierarchy set to True, when a PF MBX is excluded from serving hierarchy, it's set to have partial hierarchy, this is by design.
When you forcefully connect to such partial-hierarchy mailbox, it is Expected to see a partial hierarchy, toggling the IsExcludedFromServingHierarchy to false will invoke the sync across that PF MBX to acquire the complete hierarchy information.