In addition to the reply above, I received this response.
Photos not scanning all imported photos
You may need to break down your 10K photos into smaller groups, and import them a bit at a time. Sometimes photos will encounter a problem and abort the import without any message
For example, if on one import, it imports two photos with the same name (say in different folders) it can abort the import silently.
Breaking down the photos into smaller sets will enable you to identify problem items. If a group then errors you can break that one down into smaller groups, until you narrow it down sufficiently.
How do I break down photos that have already been imported? The import worked it seems.
There have been no additional photos scanned in People view in over 1 week.