Publishing several research papers in the same journal
It would depend on the journal, of course. There should be no negative effect if it is a high quality, respected journal such as one associated with a professional society. A predatory journal would be a different matter of course. But the several papers in the same high quality journal have been reviewed by a fairly large number of independent reviewers. 

Whether the editors would continue to accept papers from the same person is up to them, of course.