BootstrapRTE - grey background to text when changing font

The BootstrapRTE as used in the EmailTemplate module has some strange behaviour. When changing the font type, a grey background is added to the text.  It's not only a styling issue on the page. It's stored in the database as well.  We use the text as email body in the EmailTemplate module and the result is that the weird grey background is also shown in the generated email. Any suggestion how to get rid of that grey background?
1 answers

Maybe update the module to version 9.0.0:

'Updated to modeler version to 7.13 and replaced bootstrap widget with Rich Text widget.’

But furthermore if the problem still exists check out the HTML that the widget is generating. This should hold some clue's if a grey background is added.