src/CMakeLists.txt:    KF5::Crash

The general assumption is that simply linking the library is not sufficient, you also need to call KCrash::initialize() in your code. Without the call the library may not actually get linked as it is not needed and result in KCrash not actually getting enabled/used.

In libktorrent's case I'd think you want ktorrent to link KF5::Crash and then call initialize in the applications main() instead of the library. There is hardly a reason why the library would need to pull in KCrash.

For some background see this thread:
https://markmail.org/thread/zv5pheijaze72bzs
and
https://api.kde.org/frameworks/kcrash/html/namespaceKCrash.html