Multisite: Assign imported content to an existing user or create new user option is broken

[henrywright]

In a network install, the option to assign imported content to an existing user or create a new user is broken. Regardless of my choice, content is always assigned to the existing user doing the import

[MadtownLems]

I just tested this on a 5.8.3 multisite instance, and had no issues assigning content to an existing user (other than myself who was doing the import).

[kfazz]

I've had this occur if the user exists on the network but isn't assigned to the sub-site that you're importing into. In this case, the importer falls back to the logged in user.