I doubt it is developer apathy. It smells like a management issue. iOS is where the money is. I don't know anyone at apple, but here's how I imagine it is there:
Every OSX engineer probably has more bugs on their plate than can be fixed in one career, in addition to feature development tasks. The OSX team is severely understaffed both because it is not a priority at Apple, and because it is getting harder and harder to find engineers that are able to/willing to work on this kind of code - a Byzantine labyrinth of C flavors that's been accreting lines for decades.
The OSX team is in a desperate fight for recognition and status against the iOS team. They believe (and are probably right) that the only way to gain executive mind share there is to build out new visible features, no matter how useless. Witness dark theme, stacks, dynamic desktop, siri on the desktop, a steady stream of useless iPhone integrations, etc. All the troops are ordered to march relentlessly toward feature development. Stability and bug fixes be damned.
It bet it takes at least three weeks of focused effort and red tape macheteing to fix this bug and it is way down on someone's list. Even if you fix it the change won't make the next OSX key note at the annual IOScon or whatever it is called, so it won't help your career one iota.
Again - this is just how I imagine it is there. I don't really know.