Checkbox default value incorrect

Hi.

First, great plugin. I expect to install half a dozen of the great add-ons you have available. (Good business model, too. I’m jealous.)

Now then, the problem: Near the bottom of this page is a checkbox (“I have skills”) that defaults to true, even though I have no default value specified for it:
https://treelight.com/voting-advice-system/sign-the-declaration/

I tried setting the default to “no”, “No”, “NO”, and “0”, but nothing works to turn it off.

An earlier checkbox in the signup form has the value “Yes”. That is checked by default. Others have no value, and they appear unchecked. Only this one item has no default, and set still appears with a value.

I’ve done Ctrl-A + Ctrl+X in the default field, to make sure it is empty.
Settings are “Not required”, Display-column=0, Admin-column=13, Sortable=yes, CSV=yes, Persistent=no, Signup=Yes, Read-only= no.

Any idea what the problem may be?