Hi Iggy,

Thank you for reporting this issue. We have reproduced and recognized the problem with the DropDownButton when showArrowButton is enabled and setOptions() is called.

Until the fix is available, you can use the following workaround after calling setOptions():

<script>
const dropDownButton = $("#default").data("kendoDropDownButton");

dropDownButton.setOptions({
    fillMode: "flat"
});

dropDownButton.element
    .find(".k-button-arrow")
    .slice(1)
    .remove();
</script>

As shown in the following dojo: https://dojo.telerik.com/nlXrNZMK/9. This removes any duplicate arrow buttons while preserving the original one.

As a small note of gratitude for reporting this, I've updated your Telerik points.

Regards,
Rumen
Progress Telerik