Hi Andrea, I have had the same problem and I solved it by updating the Atlas core. I hope it will solve your problem as well. Good luck!
Could be the ouline when the checkbox has a focus state. Try this:
.mx-checkbox{
input:focus{
outline: none;
}
}
hii Andrea Steele
this is happening by a class just inpect that on browser and see which class is creating it and remove that class or make it display none