What have I forgotten?

[Ofnuts]

"Custom"' means you have altered/edited the gradient. When you edit the gradient in the Blend tool the edited version is renamed to "Custom" and is made the active gradient (so this replaces the standard gradient you had before using the tool for all future uses, until you explicitly re-selected another gradient.

And from what you show it seems that the midpoint of the Custom gradient has been dragged all the way to the end point, which would explain the behavior.