Hi,
We are using the latest version of Power BI Desktop - August 2020. The up and down arrows are only on the Explain By fields that are selected onto the visual, whereas I am referring to the list that appears when you press the "+" button to decide which Explain By field you wish to select.
I have put a picture below to demonstrate, I have added A-Z as Explain By fields, but in certain scenarios/screen sizes, you cannot see the entire list and there is no way to scroll. For example I opened it in the Teams client on a small screen, the list only gets as far as "P". On the iOS app, the list only gets as far as "N". Even in the Power BI Desktop screen it can be cut short on a small screen (although I haven't included a picture of this below):
Many thanks.