Blender-2.8. UV issue

Look at this.

When planar circled uv part has only one seam - it does not cutted to line ribbon
In my opinion, it became notmal only wit 2 an more cuts.
Is this a bug?

It is normal. Sides of cylinder is made of rectangular faces.
But faces of showed parts, you are talking about are trapezoidal.

UV Unwrapping tries to preserve closest area and closest angle.
So, UV face shape with closest area is also trapezoidal. And if you glue same trapezoidal face to previous one, several times, resulting ribbon is a circle.

To obtain a ribbon from that, you just have to choose one face of UV Island.
Unwrap It, align its sides to transform it into a rectangle.
Then, Unwrap UV Island using Follow Active Quads item.

Thank you for reply
But anyway, will be nice to have this option

or just install textools or any uv addon with rectify and do the same in 1 click