Use the Makefile

The wrong one that use go fmt . is editing go.mod file

$ make wrong

Correct one is that use gofmt -l -w .. No change in go.mod

$ make fine

What i see a problem is change of go.mod.

I shouldn't happend.

And another issue is that output of go fmt . and gofmt -l -w . should be consistent. See go help fmt

$ go help fmt
usage: go fmt [-n] [-x] [packages]

Fmt runs the command 'gofmt -l -w' on the packages named
by the import paths. It prints the names of the files that are modified.

For more about gofmt, see 'go doc cmd/gofmt'.
For more about specifying packages, see 'go help packages'.

The -n flag prints commands that would be executed.
The -x flag prints commands as they are executed.

To run gofmt with specific options, run gofmt itself.

See also: go fix, go vet.