You should be able to reliably reproduce this issue like this:
$ go clean -modcache
$ GOPROXY=direct go mod download -x go.mongodb.org/mongo-driver@v1.13.0
The v1.13.0 tag seems to have been “moved”, and proxy.golang.org just happens to have seen the first attempt.
The snippet above downloads the current (presumably retagged) content, but compares it to the checksum in sum.golang.org (which is the old one). So ⇒ 
You can also compare the contents of the repo at the current v1.13.0 to the zip downloaded from proxy.golang.org and verify that they indeed do differ.
@Preston_Vasquez we probably need a minor tag to fix this.