DocumentManifest searches return 0 results when using FHIR R4 'patient' parameter

[JeremiahSanders]

Describe the bug
When searching DocumentManifest resources using the patient parameter, searches return 0 results. Using the subject parameter, with the same Patient reference, returns the expected resources.

To Reproduce

Steps to reproduce the behavior:

var fhirClient = new Hl7.Fhir.Rest.FhirClient("https://spark.incendi.no/fhir/");
var patient = await fhirClient.CreateAsync(new Patient
{
	Name = new List<Hl7.Fhir.Model.HumanName> {
        HumanName.ForFamily("SampleFamily").WithGiven("SampleGiven").WithGiven("ExtraGiven")
    }
});
var patientRelativeUrl = $"{patient.TypeName}/{patient.Id}";
var documentManifest = await fhirClient.CreateAsync(new DocumentManifest
{
	Subject = new ResourceReference(patientRelativeUrl),
	Content = new List<ResourceReference>{
		new ResourceReference("DocumentReference/not-real")
	},
	Status = DocumentReferenceStatus.Current
});

// This finds the resource
var bySubject = await fhirClient.SearchUsingPostAsync<DocumentManifest>(new SearchParams("subject", patientRelativeUrl));
// These don't find the resource
var byPatient = await fhirClient.SearchUsingPostAsync<DocumentManifest>(new SearchParams("patient", patientRelativeUrl));
var byPatientId = await fhirClient.SearchUsingPostAsync<DocumentManifest>(new SearchParams("patient", patient.Id));

Expected behavior

The API is expected to return a Bundle with the same DocumentManifest resources obtained when searching by subject for the same Patient.

Related FHIR search parameters: http://www.hl7.org/fhir/documentmanifest.html#search

Spark version

• Version: 1.5.14

Operating system + Database

Validated against reference implementation at https://spark.incendi.no/fhir/.

Additionally, validated locally in Windows 11 + Docker-hosted MongoDB.

Container service / Cloud infrastructure:

Validated against reference implementation at https://spark.incendi.no/fhir/.

Additionally, validated locally in Windows 11 + local Docker Desktop-hosted MongoDB.