With Qt 6.8.1, the following code

auto url = QUrl::fromUserInput(QStringLiteral("/tmp/..///asdf/"));
auto adjusted = url.adjusted(QUrl::NormalizePathSegments);
qCritical() << url << adjusted;

prints
QUrl("file:///tmp/..///asdf/") QUrl("")

With Qt 5.15.16 (and, as far as I know, with Qt 6.8.0), the same code prints
QUrl("file:///tmp/..///asdf/") QUrl("file:///asdf/")

Qt Test prints more information about the invalid URL:
Invalid URL: Path component starts with '//' and authority is absent; source was "//asdf/"; scheme = "file", path = "//asdf/"

This regression causes a failure of KDevelop's TestPath unit test, but I am not aware of any problems for non-test code.