updated known issues

Packages/com.alelievr.mixture/Documentation~/KnownIssues.md

@@ -17,4 +17,10 @@ Fixing that would require a complete refactor of the render texture system used
 In some versions of Unity (above 2020.3.4f1 and 2021.1) the mixture package causes the preview of the texture importer to break (throw error in the console).
 You can have more details about this issue here: https://issuetracker.unity3d.com/issues/texture-importer-inspector-throws-errors-when-a-built-in-texture-inspector-is-overwritten-in-c-number
 
-The only known workaround is to downgrade to Unity 2020.3.4f1 or remove the CustomEditor for 2D mixture asset (but you loose the mixture UI in the inspector).
+The only known workaround is to downgrade to Unity 2020.3.4f1 or remove the CustomEditor for 2D mixture asset (but you loose the mixture UI in the inspector).
+
+## Memory leak after Undo/Redo operation in the editor
+
+When using the Undo/Redo operation while a Mixture graph is open, a part of the RenderTexture and ComputeBuffers allocated for the processing of the Mixture will not be cleanup correctly, causing the memory to grow linearly.
+
+This is caused by the way Unity serialize classes with the [SerializeReference] attribute, hopefully it will be fixed someday. If not, we'll make a new system to avoid having managed memory inside serialized nodes.