Additional known issue for 8.16.6 RNs (#6728) (#6736)

* Additional known issue for 8.16.6

* Applies feedback

* Update docs/release-notes/8.16.asciidoc

Co-authored-by: Janeen Mikell Roberts <[email protected]>

---------

Co-authored-by: Janeen Mikell Roberts <[email protected]>
(cherry picked from commit aecdee4)

Co-authored-by: natasha-moore-elastic <[email protected]>

Author: mergify[bot]

docs/release-notes/8.16.asciidoc

@@ -29,6 +29,19 @@ On November 12, 2024, it was discovered that manually running a custom query rul
 ====
 // end::known-issue[]
 
+// tag::known-issue[]
+[discrete]
+.Installing an {elastic-defend} integration or a new agent policy upgrades installed prebuilt rules, overwriting user-added actions and exceptions
+[%collapsible]
+====
+*Details* +
+When you install an {elastic-defend} integration or a new agent policy for this integration, all the installed prebuilt detection rules are upgraded to their latest versions (if any new versions are available). The upgraded rules lose any user-added rule actions and exceptions.
+
+*Workaround* +
+To resolve this issue, before you add an {elastic-defend} integration to a policy in {fleet}, apply any pending prebuilt rule updates. This will prevent rule actions and exceptions from being overwritten.
+====
+// end::known-issue[]
+
 [discrete]
 [[bug-fixes-8.16.6]]
 ==== Bug fixes