What happened?

1.When rebuilding the Smart Search index, the indexer always stops after the first batch when using Joomla core Finder plugins such as:

Smart Search - Content
Smart Search - Categories
Smart Search - Contacts
Smart Search - Tags

The Event Booking Finder plugin works correctly, but only because the number of indexed items is lower than the configured batch size.

The problem occurs exactly when Joomla starts the second AJAX batch request.

Steps to reproduce
Install Joomla 6.1.2.
Enable only:
Smart Search - Content
Disable all other Finder plugins.
Go to Components → Smart Search.
Set the Indexer Batch Size to 50, 100 or 300.
Start indexing.
Expected result

The indexer should continue processing all batches until every item has been indexed.

Actual result

The first batch always completes successfully.

Examples:

Batch size = 50 → exactly 50 items indexed, then stops.
Batch size = 100 → exactly 100 items indexed, then stops.
Batch size = 300 → exactly 300 items indexed, then stops.

The browser then receives:

HTTP 500

for

administrator/index.php?option=com_finder&tmpl=component&format=json&task=indexer.batch

No further batches are processed.

Additional information
The Finder tables were rebuilt from scratch.
Database structure appears correct.
Joomla Debug mode does not reveal any PHP exception.
Apache/cPanel error log contains nothing related to the failure.
The Finder log only contains:
Starting the indexer
Starting the indexer batch process
Batch completed
A "finder" plugin has had an error.

No PHP fatal error or stack trace is written.

The browser console only reports the failed AJAX request (HTTP 500).

Important observation

The issue seems related to moving from the first batch to the second batch, not to indexing itself.

If the total number of items is less than the configured batch size, indexing completes successfully.

For example:

40 items + batch size 50 → completes successfully.
300 items + batch size 300 → completes successfully.
301 items + batch size 300 → fails immediately after the first batch.

This strongly suggests that the failure occurs when Joomla initiates the next AJAX indexing request rather than while indexing the current batch.

Logs
Starting the indexer
Starting the indexer batch process
Batch completed
A "finder" plugin has had an error.
Browser

Chrome (latest)

Console
GET administrator/index.php?option=com_finder&tmpl=component&format=json&task=indexer.batch

HTTP 500 Internal Server Error

Version

6.2

Expected result

No response

Actual result

No response

System Information

Joomla version
Joomla 6.1.2
Same issue also occurred on Joomla 6.1.1
PHP version 8.3
Database

(MySQL/MariaDB )

Additional Comments

I would be happy to test a patched version or add additional logging if someone can point me to the correct location in the Smart Search batch process.