SQL Error [1253] - COLLATION 'utf8mb3_unicode_ci' is not valid for CHARACTER SET 'utf8mb4' introduced in MariaDB 10.11.11+

After upgrading AWS RDS MariaDB from 10.11.10 to 10.11.16, queries using `COLLATE utf8_unicode_ci` (utf8mb3_unicode_ci) in SQL expressions started throwing Error 1253. The same queries work fine on 10.11.10 and also on 11.5.2.

*Steps to Reproduce:*
1. Create a table with CHARSET=utf8mb3 COLLATE=utf8mb3_unicode_ci
2. Connect to server with default character_set_connection = utf8mb4
3. Run a query with COLLATE utf8_unicode_ci in an expression:

SELECT * FROM conditional_product_rule_configs

WHERE id IN (869)

AND JSON_UNQUOTE(JSON_EXTRACT(config, '$.key')) COLLATE utf8_unicode_ci = 'timeframe'

LIMIT 1;

*Expected Result:*
Query executes successfully (as it did on 10.11.10 and 11.5.2)

*Actual Result:*

SQL Error [1253] [42000]: COLLATION 'utf8mb3_unicode_ci' is not valid for CHARACTER SET 'utf8mb4'

*Environment:*

• Affected version: MariaDB 10.11.16 (AWS RDS)
• Working versions: MariaDB 10.11.10, MariaDB 11.5.2
• Server variables:
• character_set_connection = utf8mb4
• collation_connection = utf8mb4_general_ci
• character_set_server = utf8mb4
• collation_server = utf8mb4_general_ci

*Table DDL:*

CREATE TABLE `conditional_product_rule_configs` (

  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,

  `config` text DEFAULT NULL,

  PRIMARY KEY (`id`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3 COLLATE=utf8mb3_unicode_ci;

*Workaround:*
Set `init_connect = 'SET NAMES utf8mb3 COLLATE utf8mb3_unicode_ci'` in the Parameter Group, which forces the connection charset to match the query's collation.

*Notes:*

• The issue appears to be a regression introduced somewhere between 10.11.10 and 10.11.16
• The behavior was fixed/reverted in 11.5.2
• The root cause appears to be stricter collation validation against connection charset