Configurations of field instances do not migrate

Problem/Motivation

Migration of Drupal 7 site: Field instance configurations do not migrate filefield_settings. I imagine others might come across this problem.

Additionally, there is a "File Directory" field in the configuration. This is confusing because I understand filefield_paths is to take over for this core functionality when applied? I cannot locate anything about the relationship of these fields.

Steps to reproduce

Using migrate plus, running a migration of field instance configuration from a drupal 7 site. I am not aware of custom code that would disrupt these settings from being saved.

Proposed resolution

I was able to fix this by putting the following into a custom field instance migration configuration:
third_party_settings/filefield_paths:
- plugin: get
source: settings/filefield_paths

I am not sure at this point the best way to accomplish this for a contrib module, but I believe perhaps hook__migration_plugins_alter() is a way and there must be some examples out there.

Remaining tasks

User interface changes

none

API changes

Data model changes