Is this something that can be attacked by loan seekers? I know that you said the issue was only with higher interest rates, but is it possible that someone that would normally not meet the loan criteria can slide past and get a loan that they shouldn't have gotten?
The problem is NOT with only higher interest rates. The bug prevents ANY cancellation at any rate. But it is only a serious matter if you set a high rate that is not taken up by a borrower. Then your funds are stuck until Polo gets around to fixing it - and that may be months from all the chatter I'm seeing about Polo's support woes.
Borrowers automatically borrow funds at the cheapest available rate, so this should have no direct impact on them. Just beware that if lending and you find you are unable to cancel, the only way to retrieve your funds is to hope someone borrows them and then wait for them to close out their position to end the loan. And for goodness' sake, turn off Auto-Renew.