Fountain Fill ISsues

try manually picking the colors in the fountain fill tool and changing them to RGB.

Using CMYK in X6 I'm getting something similar to you, except that my pronounced band is lighter rather than darker.

But I can improve it slightly by spreading around the middle:

• Add the three colours at 0, 50 and 100% positions
• Double click at about the 45% position, but do not change the colour. Instead, drag it to the 40% position.
• Double click at about the 55% position, but do not change the colour. Instead, drag it to the 50% position.

You will see an improvement, though you may still not get a perfect blend. It probably depends in part on your graphics card and screen quality. But unless your background is intended for the web, the way it prints is probably more important -- and that too, I suspect, will vary from printer to printer.

Try using a black made from your Purple e.g. C40 M100 K100 that should help!

Below is Purple C40 M100 fading to C40 M100 K100

As Chris Wills said you have to do the transition between your color in center to a darker color but it needs not only K /black/ because in CMYK the K color or 100% black not contain any other colors in it so the result is like yours. But if you mix the basic color in center with black in percents you need you will have a nice gradient. For example if you want to make a gradient from red to black you have to start with 100% magenta + 100% yellow /it's actually red in CMYK/ and at the end of the gradient to put the same color + black /30 or 40, 50 even 100% if you need/.
In RGB it's different and you have to read a little for understanding additive adn substractive color modes, how they behave and when you need to use them. It's not a good idea to work in RGB if you need to print in CMYK, or to work in CMYK when you need something for web for example.