Salesforce DX failing on force:package:version:create: Not in package.xml error

Question

I'm working on a SFDX package within a project that has no dependencies. Every time I create a package version, it fails with the following error:

ERROR running force:package:version:create:  Hed__course_enrollment__c.Default: Not in package.xml,Course_enrollment__c.Default: Not in package.xml,Case.DefaultProcess: Not in package.xml,Case.Default: Not in package.xml,Account.Default: Not in package.xml

My package has no external dependencies, and my scratch JSON file does not reference these files... This error started appearing when I tried to create a package version for a different package version in a different package. However, now I cannot create any package versions...

Can someone please point me to any steps I can follow to resolve this error?

More details below:

I had been working with a Salesforce unlocked package directory, trying to create a package version for a package with a dependency on Salesforce EDA. During that investigation (see this question for details), I made the following change to my project-scratch-def.json file:

{
  "orgName": "Destiny Solutions",
  "edition": "Developer",
  "features": ["RecordTypes"],
  "objectSettings": {
    "account": {
        "defaultRecordType": "default"
    },
    "case" : {
      "defaultRecordType": "default"
    },
    "hed__course_enrollment__c": {
      "defaultRecordType": "default"
    }
  }
}

This caused a subsequent call to sfdx force:package:version:create to fail with the following error:

ERROR running force:package:version:create:  Course_enrollment__c.Default: Not in package.xml,Hed__course_enrollment__c.Default: Not in package.xml

After that failure, I created a branch new SFDX project with no dependencies and a single package. My project json and scratch json files are given below:

{
  "packageDirectories": [
    {
      "path": "contact",
      "default": true,
      "package": "DSConnectContact",
      "versionName": "Release 5.3.0",
      "versionDescription": "Destiny Connect Salesforce Reference for Contacts - 5.3.0",
      "versionNumber": "5.3.0.NEXT",
      "definitionFile": "config/project-scratch-def.json"
    }
  ],
  "namespace": "",
  "sfdcLoginUrl": "https://login.salesforce.com",
  "sourceApiVersion": "49.0",
  "packageAliases": {
      "DSConnectContact": "0Ho4W000000k9gJSAQ"
  }
}

{
  "orgName": "Destiny Solutions",
  "edition": "Developer",
  "features": [],
  "settings": {
    "lightningExperienceSettings": {
      "enableS1DesktopEnabled": true
    },
    "securitySettings": {
      "passwordPolicies": {
        "enableSetPasswordInApi": true
      }
    },
    "mobileSettings": {
      "enableS1EncryptedStoragePref2": false
    }
  }
}