chore: announce kafka connector group ID generation strategy change

Author: chillleader

docs/components/connectors/out-of-the-box-connectors/kafka.md

@@ -291,6 +291,7 @@ In the **Authentication** section, select the **Authentication type**. If you se
 
 In the **Kafka** section, you can configure the following properties:
 
+- **Consumer Group ID**: Set the consumer group ID for this connector. It is strongly recommended to always provide an explicit value. Use a stable, application-specific identifier that represents the logical consumer group (for example, `my-app-order-processor`). If left empty, an ID is auto-generated from the connector's internal deduplication key — this generated ID may change across connector upgrades, causing Kafka to treat the connector as a new consumer group and potentially replay already-processed messages.
 - **Schema strategy**: Select the schema strategy for your messages.
   - Select **No schema**, **Inline schema** for Avro serialization.
   - Select **Schema registry** If you have a Confluent Schema Registry.
@@ -538,6 +539,10 @@ If any of the field is not populated, you must configure your security method fo
   enable.auto.commit=false
   ```
 
+:::caution
+The `group.id` value above is auto-generated when no explicit **Consumer Group ID** is configured in the connector. This generated ID is derived from the connector's internal deduplication key, which may change across connector upgrades (for example, when upgrading from 8.8 to 8.9). A changed group ID causes Kafka to treat the connector as a new consumer group, losing committed offsets and potentially replaying messages. It is strongly recommended to always set an explicit Consumer Group ID.
+:::
+
 ### What is the precedence of client properties loading?
 
 Properties loading consists of three steps:

docs/reference/announcements-release-notes/890/890-announcements.md

@@ -543,6 +543,26 @@ For more information on optimizing connector performance with virtual threads, s
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+#### Kafka Consumer connector: auto-generated consumer group ID format changed
+
+When upgrading from Camunda 8.8 to 8.9, the auto-generated Kafka consumer group ID changes format. This is a side-effect of a runtime improvement that enables cross-version connector deduplication.
+
+- **8.8 format:** `kafka-inbound-connector--<processDefinitionKey>-<elementId>` (numeric key)
+- **8.9 format:** `kafka-inbound-connector--<bpmnProcessId>-<hash>` (stable process ID)
+
+Because Kafka treats a changed group ID as a brand-new consumer group, connectors using an auto-generated group ID after upgrading will lose their committed offsets and may reprocess messages.
+
+**Action required:** Before upgrading to 8.9, set an explicit **Consumer Group ID** in each Kafka Consumer connector configuration to preserve consumer group identity. See the [Kafka connector documentation](/components/connectors/out-of-the-box-connectors/kafka.md?kafka=inbound) for details.
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