To reproduce: 

1. Check out an item to a patron
2. In the checkouts table on that patron's account, check the "Renew" checkbox for that item
3. Select a custom renewal date using the Renewal Due Date flatpickr
4. Click Renew Selected Items
(4.5 I'm also seeing a bug at the moment where the checkouts table won't reload on its own - refresh the page if this is the case)
5. Note the item's new due date is 5 days after the previous one (or whatever renewal period is specified in the circ rules, if you are not using default KTD settings), regardless of the date you entered as the custom renewal due date

This also occurs with the "Renew All" button