Accordion: card-header border bottom hidden by card-body.

[MargotPiva]

When an accordion element is open, the border bottom of the card-header element is hidden by the card-body element (in case of non-transparent background). See here the actual comportment : https://codepen.io/anon/pen/GzjNYL.
This behavior is due to this :

bootstrap/scss/_card.scss

Line 286 in e65dccf

margin-bottom: -$card-border-width;

This addition is due to those issues : #27124, #27108.
The bug reported here was the double border-bottom between the accordion elements when closed.

See the result when removing the rule over the margin-bottom : https://codepen.io/anon/pen/XOjNGz
Unfortunately the double border-bottom is back but the behavior of the card-header of an open element seems correct with a border-bottom.

[kampalex]

Using the dark colors the latest codepen example is very hard to exam.

The border-bottom is shown on transparent background. Due to the negative margin, the border-bottom of .card-header is covered by .card-body.
By applying 'z-index:1' onto .card-header styling, the border-bottom appears again.

[florianlacreuse]

OP already said that this issue only occur in cases with with non-transparent backgrounds because of the negative margin .card-header.

Using z-index seems to do the job but i'm not sure if this is a cleanest way to fix it. No clear-cut opinion about it, just wondering since I think this should be fixed in Bootstrap project.