After connecting from your Destination server to your Source server, you have various search options inside the Transfer Tool to help you control what users are transferred. When using the dropdown menu to perform a more advanced search, the "Reseller" text field is very small, making it hard to use.
We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-40735. Follow this article to receive an email notification when a solution is published in the product.
There is no workaround available at this time.