Banding in Fountain Fill Gradient

This is a phenomenon that is more or less visible depending on the colors involved in the fill.
A fountain fill (gradient) is shown on screen as a set of colored fields (Options > Workspace > Display > "Preview fountain steps").
The default value, and as a matter of fact also the maximum value, is 256 steps.
Most of the times this will present a nice blend without visible banding, but there are also cases where these bands will be more or less visible.

There is also another important part of this, and that is how a fountain fill will print.
In the Edit Fill dialog, there is a setting for "Flow", where it is possible to change how many steps the fill will be built up of.
I know it is tempting to increase this setting to 999 steps (maximum) but do not do that.
Leaving this setting greyed out (click the "Set as default" icon next to it, if it isn't) will ensure that when you print, it will be as good as the printer can do it.
Basically, it will then tell the printer to print from color A to color B as good as you can.
Setting it to even its maximum of 999 steps can, in some rare cases, actually create very visible banding.
I have experienced this myself once in a large format print with a fountain fill that had a dark brown gradient.

Not sure how it works in Photoshop but it is possible they use another technique to show a gradient on screen.