[7.x][DOCS] Add a known issue (#96123) to 7.12 release notes (#99617) (…

…#99965)

Co-authored-by: Kaarina Tungseth <kaarina.tungseth@elastic.co>

docs/CHANGELOG.asciidoc

@@ -210,6 +210,8 @@ Refer to https://www.elastic.co/blog/discover-uses-fields-api-in-7-12[the blog a
 [[known-issues-v7.12.0]]
 === Known issues
 
+*Saved object migrations*
+
 When upgrading to 7.12.0, {kib} retries failed saved object migrations without user intervention.
 
 Impacts include:
@@ -223,6 +225,19 @@ Impacts include:
 
 If your upgrade migration fails repeatedly, refer to {kibana-ref}/upgrade-migrations.html#preventing-migration-failures[preventing migration failures].
 
+*Cluster performance degradation*
+
+Details:
+
+{kib} fails to update and delete {kibana-ref}/search-sessions.html[search sessions objects] created in non-default spaces.
+
+Impact:
+Cluster performance degrades due to a flooding of async search API usage.
+For information on the workaround for 7.12.0, refer to {kibana-issue}96124[#96124].
+
+For information on the fix in 7.12.1, refer to {kibana-pull}96123[#96123]. 
+
+
 // end::known-issues[]
 
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