Bug in the preview of the gradients in the latest version of the illustrator.
In the latest version of Illustrator 25.4.1 the gradient preview appears broken. The same file opened in the lower 2019 version of Illustrator does not have this problem.
Ralitsa Ruseva shared this idea
roma krivenko commented
Same glitch on Macbook Pro M1, Illustrator 25.4.1: https://www.dropbox.com/s/y9wllpsa58ioe2u/gpu_glitch.mov?dl=0
What is your GPU specs?