rest – Why SharePoint ListItemAllFields not returning all fields?

Using the following I can see all properties and column values for documents in my library/folder except for one.

_api/web/GetFolderByServerRelativeUrl('/mylibrary/myfolder')/Files?$expand=ListItemAllFields

Its a typical lookup column, and it exists in all my libraries.
When I try the above it shows up in the properties, whether a value is assigned or not, in every library in the site collection except one.

<d:PackageId m:null="true" />

Why? I can’t see any setting or config that makes this library different than the others.
A dozen custom columns, including lookups, but just this one doesn’t appear.