Autocomplete user search API does not partial match the current user

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

When making a query to the autocomplete API, the currently authenticated user does not show up in the results, unless the full user name is provided.

Steps to reproduce

full name (shows result)

curl -i -u admin:admin -X GET -H "OCS-APIRequest: true" 'http://nextcloud.local/ocs/v2.php/core/autocomplete/get?search=admin'

<?xml version="1.0"?>
<ocs>
 <meta>
  <status>ok</status>
  <statuscode>200</statuscode>
  <message>OK</message>
 </meta>
 <data>
  <element>
   <id>admin</id>
   <label>admin</label>
   <icon>icon-user</icon>
   <source>users</source>
   <status></status>
   <subline></subline>
   <shareWithDisplayNameUnique>admin@example.net</shareWithDisplayNameUnique>
  </element>
 </data>
</ocs>

partial name (no result)

curl -i -u admin:admin -X GET -H "OCS-APIRequest: true" 'http://nextcloud.local/ocs/v2.php/core/autocomplete/get?search=ad'

<?xml version="1.0"?>
<ocs>
 <meta>
  <status>ok</status>
  <statuscode>200</statuscode>
  <message>OK</message>
 </meta>
 <data/>
</ocs>

only happens for currently authenticated user, partial matches work for other users

curl -i -u admin:admin -X GET -H "OCS-APIRequest: true" 'http://nextcloud.local/ocs/v2.php/core/autocomplete/get?search=ja'

<?xml version="1.0"?>
<ocs>
 <meta>
  <status>ok</status>
  <statuscode>200</statuscode>
  <message>OK</message>
 </meta>
 <data>
  <element>
   <id>jane</id>
   <label>jane</label>
   <icon>icon-user</icon>
   <source>users</source>
   <status></status>
   <subline></subline>
   <shareWithDisplayNameUnique>jane</shareWithDisplayNameUnique>
  </element>
 </data>
</ocs>

Expected behavior

Partial searches should work

Nextcloud Server version

master

Operating system

Debian/Ubuntu

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

Start by reproducing the OCS autocomplete endpoint with the provided curl requests, comparing the full-name and partial-name searches for the authenticated user. Trace the server-side handling of /ocs/v2.php/core/autocomplete/get and verify that a partial search returns the current user as it already does for other users.