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The I have taken a look at the styles which you have provided. The issue appears to be that the menu control does not have style rules for the following classes - rmLeftArrowDisabled, rmTopArrowDisabled, rmBottomArrowDisabled, rmRightArrowDisabled since the default style sheet has been disabled. You can fix this if you apply the bellow style rules to your CSS styles:
This fixed the issue on my end.
All the best,
the Telerik team