Hi!
It looks like the output of the checkboxes is altered, and additional line breaks are present between them, which causes this visual bug. It might be from a custom filter from another plugin or the theme, it’s nearly impossible to tell.
Instead of investigating, I rather recommend a much simpler solution. Just use this custom CSS:
[code].option input[type=checkbox]:checked ~ label:after {
opacity: 1 !important;
}[/code]
That should do the trick.