The jsonnetfmt formatter doesn't work at all, fails with an error about passing -path as an argument:

Error formatting file: ERROR(ShellError): Error running: jsonnetfmt -path somepath.jsonnet ERROR: unrecognized argument: -p 

Looks like it was just never working. I can't find any evidence that jsonnetfmt ever supported a --path argument. That seems to have just been copypasta from another formatter.