issue in fountain fill

Have you set the number of Fountain steps to a fixed value?
Do not do that, ever!

As long as you leave the padlock locked, the fill will be sent to the RIP/printer as "create a fill going from color A to color B", and the printer will create the fill as smooth as it possibly can.
If you unlock the padlock and type a value instead, it will force the fill to a limited number of steps.
And even if you type 999 steps, it may be much less depending on how the fill built up.

I once had a dark brown radial fountain fill where the only difference between the end colors was around 20% more black in the darker one.
When printed it looked awful with clearly visible banding, this even though I had it set to 999 steps.
It turned out that since the only difference in color was the 20% in black, it created the fill with 21 steps instead of 999.
After locking the padlock and letting the RIP take care of the fill, it came out super smooth.