Cylinder not cylindrical

I have a Zmorph Fab which seems to work well.
But when PLA printing a horizontal cylinder, the top is perfect while the bottom is a disaster!

I tried with the single nozzle and with the dual pro, same result.

I tested several settings of support, filling, it is always the same result: the bottom is not cylindrical.
I can’t print vertical because my other prints are going to be tee-shaped.

Any ideas to improve this?
thanks in advance