Issue with customized resource names and getRelatableResourceTypesByField

I got more clarity on this after my original post, so I'm adding the TL/DR version here:

Steps to recreate:

1. Drupal site (try Standard profile) using jsonapi and jsonapi_extras
2. Node type with an entityreference field (built-in with Standard profile: article + tags)
3. In jsonapi_extras config, override the node type and change the "alias" of the entityreference field to something else (eg, from "field_tags" to just "tags")
4. In jsonapi_extras config, override the referenced entity/bundle and change the "resource type" to something else (eg, from "taxonomy_term--tags" to just "tags")
5. Create a node, referencing an entity through this field (ie, create a tag term and then an article referencing that tag)
6. Delete the referenced entity (the tag)
7. Try a jsonapi query that would grab the node, along with the referenced entity (eg: /jsonapi/node/article?include=tags&fields[node--article]=title,tags&fields[tags]=name)
8. See the AssertionError mentioned below

Proposed solution:

When a referenced entity is missing/null, getRelatableResourceTypesByField is being called with $field->getName(), which is always the resource's internal name. So in the attached patch I'm explicitly converting that to the resource's public name.

Original post follows:

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I don't know whether this is a bug with this module, or a bug with my site, or simply a support request. Apologies for my ignorance here, but you gotta start somewhere right? Either way I could use some help here:

First here are my versions:

• jsonapi: in core, Drupal 8.7.3
• jsonapi_extras: 3.8.0

I am requesting data with this URL: /api/agency_components?fields[agency_component]=title,agency,submission_address,submission_fax,website,foia_officers,public_liaisons,service_centers&fields[agency]=name&fields[foia_officers]=name,title,email,phone&fields[public_liaisons]=name,title,email,phone&fields[service_centers]=name,title,email,phone&include=agency,foia_officers,public_liaisons,service_centers&page[limit]=50

I am getting this error:
AssertionError: assert(is_string($resource_type) || $resource_type instanceof ResourceType) in assert() (line 81 of core/modules/jsonapi/src/JsonApiResource/ResourceIdentifier.php).

From some xdebugging it seems like this is resulting from an empty array being returned by getRelatableResourceTypesByField. The getRelatableResourceTypes method returns an array of field names that have been customized by this module. For example, instead of "field_service_centers" it has "service_centers". But $field_name, at this point in the code, is 'field_service_centers'. This means that isset($relatable_resource_types[$field_name]) is always false when it runs on a field that has had its name customized by this module.

Hope that makes sense. Please let me know if you think this is due to a quirk or misconfiguration in my site, and I will try to recreate with a blank slate Drupal site. Thank you!