Bug Error: Invalid SortingGroup index set in Renderer

  SortingGroup[] sortingGroups;
  SortingGroup[] SortingGroups => sortingGroups ?? (sortingGroups = GetComponentsInChildren<SortingGroup>());
 
    void OnEnable()
    {
        foreach (SortingGroup sortingGroup in SortingGroups)
            sortingGroup.enabled = true;
    }
 
    void OnDisable()
    {
        foreach (SortingGroup sortingGroup in SortingGroups)
            sortingGroup.enabled = false;
    }

The script with the similar code should be attached to the root of pooled object.
Hope, it will help you.

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Still happening in 2020.1.10 ... looks like this is caused, for me, by creating an object from a pool and setting one of its children to active in the same frame.

Happens very infrequently, added a sorting group to the parent object in an attempt to hack around this, will update if this doesn't work.