Document Set Content Type mapping - users not being mapped

Hi there, 

I am migrating content from SharePoint 2010 on premise to SPO. I am copying over content types, and choose "Apply new content types during copy", then choose just items and not folders. 

The data is migrating fine and columns mapping perfectly except for one field - the PM field which is a user field. 

It is just not being populated. It's also not available on the list of destination fields if I choose "Map source data to new content type columns"

Modified by and Created by fields are mapping fine as I have a domain mapping setting to ensure on premise users are being mapped to spo. 

Any ideas why this wouldn't be mapping?