As the comment says, the OS does support the flag and a perusal of the source code for dyld[1] shows it does handle the flag, setting an internal flag called "leaveMapped", which is different from the "neverUnload" flag. My guess is that it literally just leaves the memory for the plugin mapped, but removes all links to it in the dyld internal data structures. That results in the Objective C runtime crashing when running content from NODELETE plugins that has been dlclose()d. This probably explains why lldb stops seeing those symbols too: (lldb) f frame #0: 0x0000000103872eb4 QtCore`QLibraryPrivate::unload_sys(this=0x000060000179c210) at qlibrary_unix.cpp:258:21 -> 258 if (doTryUnload && dlclose(pHnd.loadAcquire())) { (lldb) target modules list libqcocoa.dylib [ 0] 81EB58D5-8D31-333A-9E8C-F2385F6EFCF4 0x00000001043de000 libqcocoa.dylib (lldb) image lookup -vn QCocoaEventDispatcher::QCocoaEventDispatcher 2 matches found in libqcocoa.dylib: Address: libqcocoa.dylib[0x0000000000036480] (libqcocoa.dylib.__TEXT.__text + 190368) Summary: libqcocoa.dylib`QCocoaEventDispatcher::QCocoaEventDispatcher(QObject*) at qcocoaeventdispatcher.mm:776 ... (lldb) n (lldb) target modules list libqcocoa.dylib [ 0] 81EB58D5-8D31-333A-9E8C-F2385F6EFCF4 libqcocoa.dylib[0x0000000000000000] libqcocoa.dylib (lldb) image lookup -vn QCocoaEventDispatcher::QCocoaEventDispatcher [1] https://github.com/apple-oss-distributions/dyld Task-number: QTBUG-135044 Task-number: QTBUG-134080 Task-number: QTBUG-133861 Task-number: QTBUG-132697 Task-number: QTBUG-102984 Task-number: QTBUG-132381 Pick-to: 6.9 6.9.0 6.8 Change-Id: I7da3b3615a6ace7c72d1fffd5cf560d8f8a4e1bb Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>