The issue with the missing bottom border was due to a particular aspect of Bootstrap's styling:
https://i.stack.imgur.com/TllfY.png
Normally, this behavior is acceptable when using the accordion without any gaps between cards. However, in my case, there were noticeable spaces between the cards resulting in the absence of the bottom border.
To address this without altering Bootstrap's core code (which would be reset with future updates), I decided to override the specific rule causing the problem:
.accordion > .card {
border-bottom: 1px solid #dddddd !important;
border-bottom-left-radius: 0.25rem;
border-bottom-right-radius: 0.25rem;
}
After making this adjustment, the layout now appears as intended:
https://i.stack.imgur.com/ZQaf6.png