Dark pixel glitch with multiple response

[cww3]

I'm having a strange problem with my multiple response questionnaire. I'm attaching test.exp which has a testintro1.que and a choice1.que. Testintro1.que only has an instruction item. Choice1.que has 3 multiple response items.

When I run test.exp, several dark pixels appear under the first button for all of the items in choice1.que. It happens with the instruction text (in testintro1.que) in particular, even when I change the spacing or delete a word. When I changed the instruction text to:

a
b
c
d
e
f

the dark pixels went away. Does anyone have any idea why the content of the instructions would matter? It doesn't happen with scale response, but this questionnaire is multiple response because we want to prevent the participants from clicking through the questions rapidly. Alternatively, is there a way for the cursor to appear in a different location after a button is clicked so that the participant can't just click again? The only option we thought of is to change the location of the responses from question to question, but is there any other option?

Thank you very much!!