DelimiterAutoCompleter working inconsistently with SyntaxEditor in DataGridView

I have the Syntax Editor in a DataGridView, so on instance is created then activated for whatever row in the DGV is selected.   I'm observing that the delimiter auto completion is working fine sometimes, but not others, as I select different cells that have the editor.   The construction of the editor occurs only once and that is where I set up the DelimiterAutoCompleter, so it seems like it should continue to work the regardless of what row I select.    Here is what my code looks like:

internal class SyntaxEditorInternal : ElementHost
{
    private static DelimiterAutoCompleter _delimiterAutoCompleter = new DelimiterAutoCompleter
   {
        CanCompleteAngleBraces = true,
        CanCompleteCurlyBraces = true,
        CanCompleteDoubleQuotes = true,
        CanCompleteParentheses = true,
        CanCompleteSingleQuotes = false,
        CanCompleteSquareBraces = true
   };

   // ...

   public SyntaxEditorInternal()
   {
      var syntaxEditor = new SyntaxEditor();
      Child = syntaxEditorWrapper;
      syntaxEditor .Document.Language = new CmdVarSyntaxLanguage();
      syntaxEditor.IsDelimiterAutoCompleteEnabled = true; 
      syntaxEditor .Document.Language.RegisterDelimiterAutoCompleter(_delimiterAutoCompleter);
      // ...
   }

   // ...
}

So, whenever the editor is created, the DelimiterAutoCompleter is registered and it is never unregistered.

What's a good way for me to investigate why the auto completion works in some cases and not in others?    It seems like it should just be a matter of whether the service is registered and IsDelimiterAutoCompleteEnabled being true.   Are there any other things that can affect it?