Steps to reproduce the issue

Add a new Field of type List to your Article. Select a value and save the article.

Get the Article over the Webservices API

GET /api/index.php/v1/content/articles/{{yourArticleId}}

Expected result

"Fieldname": {
"1": "1"
},

Actual result

"Fieldname": [0]

System information (as much as possible)

PHP Built On Linux dd51504 5.4.0-164-generic #181-Ubuntu SMP Fri Sep 1 13:41:22 UTC 2023 x86_64
Database Type mysql
Database Version 10.5.22-MariaDB-1:10.5.22+maria~ubu2004-log
Database Collation utf8mb4_general_ci
Database Connection Collation utf8mb4_general_ci
Database Connection Encryption None
Database Server Supports Connection Encryption Yes
PHP Version 8.1.22
Web Server Apache
WebServer to PHP Interface fpm-fcgi
Joomla! Version Joomla! 5.0.0 Stable [ Kuboresha ] 17-October-2023 16:00 GMT
Joomla Backward Compatibility Plugin Enabled ()
User Agent Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/119.0.0.0 Safari/537.36

Additional comments

This was seen and reported in Release 4.4.0

See also fixed issues 42200 and 42206 for J4

Most likely the same issue.