Add known issue for stuck agent upgrade (8.12.0, 8.12.1)

Author: kilfoyle

docs/en/ingest-management/release-notes/release-notes-8.12.asciidoc

@@ -132,6 +132,50 @@ So, if you are relying on an integration level custom ingest pipeline introduced
 Refer to the <<data-streams-pipelines,Ingest pipelines>> documentation for details and examples.
 ====
 
+[discrete]
+[[known-issues-8.12.1]]
+=== Known issues
+
+[[known-issue-3263-8121]]
+.Agents upgraded to 8.12.0 are stuck in a non-upgradeable state
+[%collapsible]
+====
+
+*Details*
+
+An issue discovered in {fleet-server} prevents {agents} that have been upgraded to version 8.12.0 from being upgraded again, using the {fleet} UI, to version 8.12.1 or higher.
+
+*Impact* +
+
+As a workaround, we recommend you to use the {kib} {fleet} API to upgrade these agents using the `force` flag.
+
+To upgrade a single {agent}:
+
+[source,"shell"]
+----
+POST kbn:/api/fleet/agents/<agent_id>/upgrade
+{
+  "version": "8.12.1",
+  "force": true
+}
+----
+
+To bulk upgrade a set of {agents}:
+
+[source,"shell"]
+----
+POST kbn:/api/fleet/agents/bulk_upgrade
+  {
+    "agents": "agent.version:8.12.0",
+    "version": "8.12.1",
+    "force": true
+  }
+----
+
+This issue is planned to be fixed in versions 8.12.2 and 8.13.0.
+
+====
+
 [discrete]
 [[bug-fixes-8.12.1]]
 === Bug fixes
@@ -405,6 +449,46 @@ Note that according to our <<agent-policy-scaling-recommendations,policy scaling
 
 ====
 
+[[known-issue-3263-8120]]
+.Agents upgraded to 8.12.0 are stuck in a non-upgradeable state
+[%collapsible]
+====
+
+*Details*
+
+An issue discovered in {fleet-server} prevents {agents} that have been upgraded to version 8.12.0 from being upgraded again, using the {fleet} UI, to version 8.12.1 or higher.
+
+*Impact* +
+
+As a workaround, we recommend you to use the {kib} {fleet} API to upgrade these agents using the `force` flag.
+
+To upgrade a single {agent}:
+
+[source,"shell"]
+----
+POST kbn:/api/fleet/agents/<agent_id>/upgrade
+{
+  "version": "8.12.1",
+  "force": true
+}
+----
+
+To bulk upgrade a set of {agents}:
+
+[source,"shell"]
+----
+POST kbn:/api/fleet/agents/bulk_upgrade
+  {
+    "agents": "agent.version:8.12.0",
+    "version": "8.12.1",
+    "force": true
+  }
+----
+
+This issue is planned to be fixed in versions 8.12.2 and 8.13.0.
+
+====
+
 [discrete]
 [[new-features-8.12.0]]
 === New features