Submitted on customer behalf:
We are currently using version 2017.3.1017.40 and noticed an issue with the display of the RadSplitButtonElement in combination with the theme Office2010Blue.
When we disable the RadSplitButtonElement, or more precisely the ActionButtonElement in the DropDownEditorLayoutPanel, it does not display as disabled in the Office2010Blue theme.
In other themes (e.g. Office2010Silver) it does show as disabled.
When trying the latest version (2020.2.616.40) we still get this behaviour, see example below:
It appears similar to the issue mentioned in https://feedback.telerik.com/winforms/1370576-fix-radcommandbar-some-of-the-elements-in-the-control-are-not-rendered-as-disabled-with-office2010blue-and-office2010black-themes