checkbox before stretching with css
1
In the CSS code in the input[type='checkbox'] {} section, try using min-width: 20px; rather than width: 20px;.
Worked for me in a test implementation with copy/pasted code although I had to set max-width in the properties of the opt-in div to test it.
I don't have a technical explanation but I believe it has something to do with the relative position or the display: flex overriding the specified width/height in pixels.