doubt about ucs changing automatically

[Tonyinfo]

in practice I set ucs on global as you see from this image: but when they select one of the 3 pink polylines and ironing, the uucs changes (see img below): why???? ? How do I leave the bird unaltered when I iron the polylinea?

 

[Cristallo]

remove dynamic ucts

 

[Tonyinfo]

already tried... but nothing.

I solved by redesigning polylines, after setting ucs as global. . But I would like to understand the functioning of this blessed ucs.