Nightshift keeps turning itself off

Question

No matter how many times I turn on Nightshift, it keeps turning itself off.

I only ever use the "custom" time range, not the "by timezone" range. Perhaps there is a bug with this setting, instead of location-based timezone?

I currently have it set to 7:09pm to 7:00am

10.14.5 (18F203) but the problem has been happening for most of Mojave, if not all of it.

I haven't reinstalled macOS or made an OS update during this time. I may have rebooted – but I can't understand why rebooting should disable my preference setting!