It turns out that this isn't a recent issue, but one that some people in the animation field have whispered about for years. I don’t know remotely enough to know the truth. There are certainly a lot of overlaps -- the opening two paragraphs of the article by Hannah Denham appear damning --
A comical warthog and wise baboon. An evil lion with a deformed eye and hyena henchmen. A lion cub that experiences profound loss, grows up under the tutelage of a talking bird, then reclaims his throne and his legacy.
It sounds like the story of Simba in the Disney classic, “The Lion King.” But legal experts, animators and anime historians say it’s more an appropriation than homage to “Kimba the White Lion,” a Japanese anime series that NBC syndicated in the United States in the 1960s.
-- as does the fact that the Japanese artist Osamu Tezuka and Walt Disney met in 1964, and Disney had expressed interest in adapting an earlier work by Tezuka. However, after reading the full piece, the sense I get is that the movie was not plagiarized in our legal sense (and in that of Japanese law and culture which are different, which in large part is why no one ever sued), but that perhaps there were some people who did know of the earlier work and may have possibly “inspired” them in part. Or not.
Oddly, for all the comments from animation experts and historians, I find a quote by of all people Matthew Broderick -- who did the voice of the grown-up Simba in the original film -- the most interesting in the article, not as proof of anything, but showing that awareness of the Japanese TV series was not limited.
You can read the article here.