If I understand correctly, I think go mod why -m <module-path> should always show a shortest path unless the target module doesn't exist in the import graph. But it sometimes shows unexpected (main module does not need ...), even if the target module definitely exists in the import graph (can be proven by go mod graph).

Try this (from #36423 (comment)):

$ git clone -q https://github.com/appleboy/gin-jwt.git && cd gin-jwt
$ go mod graph | grep 'github.com/belogik/goes'
github.com/astaxie/beego@v1.11.1 github.com/belogik/goes@v0.0.0-20151229125003-e54d722c3aff
$ go mod why -m github.com/belogik/goes
# github.com/belogik/goes
(main module does not need module github.com/belogik/goes)

I think the expected output of go mod why -m github.com/belogik/goes here should be:

# github.com/belogik/goes
github.com/appleboy/gin-jwt/v2
github.com/appleboy/gin-jwt/v2.test
github.com/appleboy/gofight/v2
github.com/appleboy/gofight/v2/example
github.com/astaxie/beego
github.com/astaxie/beego/logs/es
github.com/belogik/goes

With this result, we can clearly know exactly how a module enters our import graph. But now we have to know it by using some workarounds like go mod graph | grep '<module-path>', this's inconvenient.

/cc @bcmills, @jayconrod