The problem here is similar to that of QTBUG-61092 only that one is claimed to have been fixed in 5.9.2 but I can see the problem in 5.10.0: tab close buttons are not rendered on macOS only in quite specific conditions: when QTabBar is used along with QTabWidget which is inside QSplitter which has some stylesheet applied. It seems just any stylesheet applied to QSplitter makes tab close buttons disappear.

Attached the test project with which the issue reproduces and the screenshot from Qt Designer - tab close buttons are not rendered even there, same thing in runtime.