Hi Rob,
you know what, i have this issue as well on OSX and i think it's been there in all 1.85 versions of pixinsight. i thought i was just losing my mind... but it does seem that PI forgets the custom swap file location on every restart, and resets back to /tmp.
I cannot reproduce this problem on any platform, including macOS (10.10, 10.11, 10.12). The selected swap storage directories are always restored on each application execution on all of our machines.
but the default location still seems to be /tmp.
On macOS we have an additional problem: the system temporary directory (on /var/folders) may change arbitrarily, typically after a few days. If you open the Preferences tool and click the Reset icon, you'll see that the current system tmp directory is set automatically on the Directories and Network section. The selected directory will be remembered. However, this is not a good solution because, as noted, the selected directory may not be available after some time.
I'll fix this issue on the next version with a special 'meta folder' that will always refer to the current system temporary directory, irrespective of its actual location. This is still not implemented in the current 1.8.5 version.