Why border is not visible with “position: sticky” when background-color exists?

I faced the same problem. My workaround was to use the :after pseudo element to emulate a bottom border.

th:after{
  content:'';
  position:absolute;
  left: 0;
  bottom: 0;
  width:100%;
  border-bottom: 1px solid rgba(0,0,0,0.12);
}