Another ACF question

Hi all, I have created a plug-in that seems to be having a problem with my class being stripped out of the page by ACF, this is the allowedContent bit from within my plugin.js that doesn't work 

var createCommand = editor.addCommand( 'createfield', 
                new CKEDITOR.dialogCommand( 'createfield', {
               allowedContent: 'span[!title](' + editor.config.fieldMarkerClassName + ')'
                } )
            );

But if I use extraAllowedContent, as below, it works fine.

config.extraAllowedContent = 'span[title](' + config.fieldMarkerClassName + ')';

Any ideas what I'm doing wrong?

BTW the varable used is defined as follows:

config.fieldMarkerClassName = 'cke-meta-field';

I have also tried hard coding the variable as I thought it may have been a timing issue, but it still didn't work, i.e. I have it like this:

var createCommand = editor.addCommand( 'createfield', 
	new CKEDITOR.dialogCommand( 'createfield', {
	allowedContent: 'span[!title](cke-meta-field)'
	} ) 
);

Thanks in advance
Mark