I made an example appending the popup of the DropDownList to a custom element and it was appended as expected:
I can suggest checking the following:
1) The element that is passed to the appendTo property is already available when the property is set.
2) Using the latest version of the '@progress/kendo-react-dropdowns' package as this prop was introduced recently.
If the issue still occurs, please share an example and I will be happy to investigate further.
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