The problem is easily reproduced using the widget demo page https://demos.telerik.com/kendo-ui/dropdowntree/index
Simply switch to the High Contrast theme; the k-widget element is lacking the border-radius style hence looks different to the other widgets.
It looks like the k-dropdowntree class is missing from the following style rule:
.k-autocomplete, .k-block, .k-calendar-container, .k-colorpicker, .k-combobox, .k-datepicker, .k-datetimepicker, .k-drag-clue, .k-dropdown, .k-dropdown-wrap, .k-editor-inline, .k-grid .k-filter-options, .k-grouping-header .k-group-indicator, .k-inline-block, .k-list-container, .k-multiselect, .k-notification, .k-numeric-wrap, .k-numerictextbox, .k-picker-wrap, .k-slider-selection, .k-slider-track, .k-split-button, .k-textbox, .k-tile, .k-timepicker, .k-tooltip, .k-touch-scrollbar, .k-treeview .k-in, .k-upload, .k-window, .k-window-action, .k-window-titleless .k-window-content {
border-radius: 13px
}