Originally the issue was related to heavy cascades within the styles and styles from horizontal menus overflowing in vertical menus and vice versa. However, with Telerik UI no longer supporting IE6 browser, we were able to use restrictive cascades (namely direct descendant selectors) that target only the right separators.
The fix was included in the release build (precursor of 2014 Q2 release), but I am still waiting confirmation when it's going to be deployed. My guess is later today or tomorrow.
In addition to fixing this bug we'll continue to improve RadMenu as it is quite burdened by overflowing selectors.
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