I see that the exact same "bug" was reported at the start of 2016 by
David Bremner.  Personally, I feel that fewer people would be confused
by this if the default value was set to "untilnow".

I also realised that the full list of values that I can use for
org-clock-display-default-range is described in org-clock.el.

By the way, you guys are brilliant and I thank you for your work on
org mode (which I'd struggle to live without now that I'm so dependent
on it).