What happened?

First test

1. Create a new field of type Media (media)
2. Create a new field of type Subform and set the Media field there.
3. Go to an article and check whether the field can be saved with content.

Second test

1. Now create a new field of type Media (document), Media (audio) or Media (video).
2. Go to the existing subform and add one or more of the new fields.
3. Create a new post and fill in the fields in the subform.
4. Saving results in an error.

Version

6.1

Expected result

The save is successful and the fields continue to display correctly afterwards.

Actual result

System Information

version: Joomla! 6.1.1-rc1 Release Candidate [ Nyota ] 15-May-2026 08:15 UTC
compatpluginenabled: false
compatpluginparameters: classes_aliases:"0", legacy_classes:"1"
phpversion: 8.4.20
php: Darwin MacBookPro.localdomain 25.3.0 Darwin Kernel Version 25.3.0: Wed Jan 28 20:54:46 PST 2026; root:xnu-12377.91.3~2/RELEASE_ARM64_T6000 arm64
dbserver: mysql
dbversion: 12.2.2-MariaDB
dbcollation: utf8mb4_unicode_ci
dbconnectioncollation: utf8mb4_uca1400_ai_ci
dbconnectionencryption:
dbconnencryptsupported: true
server: Apache/2.4.67 (Unix) OpenSSL/3.6.2
sapi_name: fpm-fcgi
useragent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/148.0.0.0 Safari/537.36

Additional Comments

Without subform, all fields work and the values are saved.