Turns out, PyTorch Lightning had nothing to do with this at all. Even just a normal vanilla PyTorch loop was causing issues terminating the program. An os._exit(0) works, but the more permanent solution I found was to update my PyTorch installation to the nightly build.

pip3 install --pre torch torchvision torchaudio --index-url https://download.pytorch.org/whl/nightly/cpu

At the time of writing, this installs a dev-build of the next PyTorch version of 2.8, which seems to have solved the issue. I do not know the root of the issue, but it's possible that the new M4 Max chip + macOS 15.5 probably caused some bugs to surface with how PyTorch terminates multiprocessing.