Currently, I can only try to make assumptions on the configuration of the project and due to that I am unable to investigate properly this problem.
From the description pf the behavior, I assume that the problem stems from the custom stylization of the Page and the Content Area of the editor. Under Chrome and Safari, the command for inline formatting are used via the browser commands for text editing and due to that they depend on the page's stylization. What is happening is that the generated span element is getting the default value of the stylization and not the content area's one.
You can resolve the issue by setting an empty EditorCssFile instance in the CssFiles collection, to clear the inherited styles:
Furthermore I would suggest you examining this help article
about custom stylization of the editor's content.
If you need further assistance on this, please open a formal support ticket with more details about the project implementation.
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