posting fixes is hardly stealing. I can agree that it could be considered (by some people) bad practice to not remove it once the original group uploaded their own fixed version, but its still a public service if anything. If one want to compare it to something, it would not be theft or copyright infringement, but rather to forking and committing a fix. And taking it down once the original has the push request accepted might be iffy, as someone might have your fork as a dependency by now. Probably safer to leave it up unless the original creator explicitly asks you to delete the fork.
I doubt ppl create dependencies on mangadex chapters though, so taking your fork down once the original is fixed is probably for the better here.