Earlier this spring, the 64th episode of the Sun & Moon anime aired in Japan. Since then, the episode’s English dub has not been broadcasted in the U.S. Rather, it was skipped entirely.
As Bulbapedia and Gamerant (via Yurukuyaru) explain, Ash dresses up like a Passimian in the episode.
However, it’s not merely that he dresses up like a Passimian that appears to have caused the ban, but his face is painted as can see in these screenshots from website Shinobu.
According to Bulbapedia, it seems as though the English-language dub of this episode wasn’t released in order to avoid possible blackface comparisons and criticism.
There is a precedent. During the 2000s, the character Jynx had to be redesigned after the character’s appearance was criticized as negative racial stereotypes.