It seems that the checkbox to decline a premium (after donating a fixed amount that qualifies for a premium) isn't enabled, after the user chooses to donate the required amount, or if that is the default donation. See attached image from latest WP sandbox. I tested this July 10, 2019.
This is still a problem. The premiums show up and are selectable - but the decline checkbox is disabled. You can select it though. Even after the required amount to donate is selected it still shows up as disabled: (I'm using version 5.41.1 on Joomla) (inspecting code shows input type="checkbox" checked="checked" disabled="disabled" )
I believe that this is hardcoded in the PremiumBlock.tpl file (lines 44 and 47). I just deleted the part that is disabled="disabled" for both lines. It works the same - but looks better.