As a result of fixing the T505420 issue, a new value of the Code property is now generated on each postback prior to the Page_Load event. This prevents submitting a form with the same Captcha code multiple times.
This breaking change will affect your code only if you verify the user input in a custom manner by programmatically comparing it with the Code property value. As a workaround, move your custom validation logic from the Page_Load to Page_Init event handler.