Doc: Add Known issue for 8.18.7: Agent stuck on failed upgrade (#1856)

Author: karenzone

docs/en/ingest-management/release-notes/release-notes-8.18.asciidoc

@@ -33,6 +33,45 @@ Also see:
 [[release-notes-8.18.7]]
 == {fleet} and {agent} 8.18.7
 
+[discrete]
+[[known-issues-8.18.7]]
+=== Known issues
+
+[[known-issue-2978-8-18-7]]
+.Failed upgrades leave {agent} stuck until restart
+[%collapsible]
+====
+
+This known issue applies to {agent} 8.18.7 and 9.0.7. {agent} versions 8.19.x and 9.1.x are not affected.
+
+On September 17, 2025, a known issue was discovered that can cause {agent} upgrades to get stuck if an upgrade attempt fails under specific conditions. This happens because the coordinator's `overrideState` remains set, leaving the agent in a state that appears to be upgrading.
+
+**Conditions**
+
+This issue is triggered if the upgrade fails during one of the early checks inside `Coordinator.Upgrade`, for example:
+
+- The agent is not upgradeable
+- Capabilities check denies the upgrade
+- When {agent} is tamper-protected, Endpoint must validate that the upgrade action was correctly signed by Kibana to allow the upgrade. If the signature is missing, invalid, or the connection between {agent} and Endpoint was interrupted, the validation fails. This causes the agent coordinator's override state to become stuck until the agent is restarted.
+
+**Symptoms**
+
+- {fleet} shows the upgrade action in progress, even though the upgrade remains stuck
+- No further upgrade attempts succeed 
+- Elastic Agent status shows an override state indicating upgrade 
+
+**Workaround**
+
+Restart the {agent} to clear the coordinator's `overrideState` and allow new upgrade attempts to proceed.
+
+**Resolution**
+
+This issue was fixed in link:https://github.com/elastic/elastic-agent/pull/9992[#9992], which ensures that the coordinator clears its override state whenever an early failure occurs.
+
+The fix is included in versions 9.1.4 and 8.19.4, and planned for versions 9.0.8 and 8.18.8.
+
+====
+
 [discrete]
 [[features-enhancements-8.18.7]]
 === New features and enhancements