The code prefilling 'no thank you reason' is not working properly. I think this was already broken for the last few months & the upgrade made me spot it (based on when the CiviCRM code actually changed)
However the question I have for @NNichols & @MDemosWMF is whether I should fix it or remove it - I feel like the business requirement has changed.
What the code should do is fill in the No Thank You Reason when a user enters a new contribution with either 'Manually entered' or 'Stock', if the financial type is Stock. Entering this prevents an automated thank you from being sent.