I was using the Bootstrap 4 .input-group-prepend class on a custom admin setting and it was behaviing oddly at narrow sizes, rather than shrinking as if one component, it was breaking across two lines. I traced it back to a line of CSS setting the width of text inputs with size to "auto" which was added in 3.2 for RTL reasons.
MDL-57524 is the RTL issue, and there was another issue about the same thing, just that the inputs were too wide.
Those changes were made before Bootstrap 4 final and it's adoption of flexbox though.
I think the Bootstrap 4 solution for this might be to wrap the items in .col-auto if you want them to auto-size based on the size attribute:
I don't think any core code is affected by this issue, the only use of the -append and -prepend classes was in search templates that aren't used in admin settngs.