See thread here codecov/feedback#726
JSON simplecov report gets matched by VOneProcessor before SimplecovProcessor can run. The report then fails to parse later because it's not a V1 report.
Edit: Simplecov is not necessarily JSON. HTML is the default format, but many formats are supported. Our current docs suggest cobertura and lcov with simplecov, and both of these seem to work. This issue concerns specifically json. See the above thread for details.