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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.