Css transition on some border property of pseudo element (eg. border-color or border-width) results in ruined flex layout. Behavior documented in codepen: https://codepen.io/t100/pen/mdRYXZw (clicking on label should not change elements arrangement, unfortunately that's not true for Safari and Chrome on iOS 14.5).
<rdar://problem/77422521>
This appears to be already fixed on ToT.
This was fixed in r275277 for bug 223990. *** This bug has been marked as a duplicate of bug 223990 ***
This should be fixed in the existing iOS 14.6 beta or an upcoming one.