Issue with styling labels

I had just learned how to style checkbox buttons and now am trying to put my new skill to use. I’ve made this pen:

https://codepen.io/Glaurung/pen/YzPEpxR

…and was expecting the checkbox to look something like this:

Checkbox1

…but got something like this:

Checkbox2

It seems like the border got sliced in half and put on two different lines. Can you please tell me what i am possibly doing wrong? What’s happening here?

Thanks.