[Direct PR] [release-18.0]: Add sidecardb known issue (#17096)

Signed-off-by: Manan Gupta <[email protected]>

changelog/18.0/18.0.0/summary.md

@@ -2,6 +2,8 @@
 
 ### Table of Contents
 
+- **[Known Issues](#known-issues)**
+  - **[VTTablet runs an unnecessary DDL](#sidecardb-issue)**
 - **[Major Changes](#major-changes)**
   - **[Breaking Changes](#breaking-changes)**
     - [Local examples now use etcd v3 storage and API](#local-examples-etcd-v3)
@@ -36,6 +38,15 @@
   - **[Durability Policies](#durability-policies)**
     - [New Durability Policies](#new-durability-policies)
 
+## <a id="known-issues"/>Known Issues
+
+### <a id="sidecardb-issue"/>VTTablet runs an unnecessary DDL
+In this release, with MySQL 8.0 whenever VTTablet's tabletserver goes through initialization, it finds
+a diff in its internal `schemacopy` table and tries to apply a DDL to fix it. The DDL
+is a no-op and this loop of applying the DDL continues to run.
+
+If this problem is encountered, the following PR should be reverted to resolve it - https://github.com/vitessio/vitess/pull/15859.
+
 ## <a id="major-changes"/>Major Changes
 
 ### <a id="breaking-changes"/>Breaking Changes