Golang -coverprofile excluding untested files from coverage percentage

I'm trying to run a test coverage report on my project which contains several directories (I'm aware this is an unusual setup but it has lots of examples which I want to group together):

└── example
    ├── bar
    │   └── main.go
    ├── baz
    │   └── main.go
    ├── foo
    │   ├── main.go
    │   └── main_test.go
    └── qux
        └── main.go

However when it run I get some strange output. The coverage percentage seems to only related to the only tested file - not the entire codebase under example.

For instance:

$ go test -race -v -coverprofile .test-coverage.txt ./...
?       github.com/abcdef/example/bar    [no test files]
?       github.com/abcdef/example/baz      [no test files]
=== RUN   TestSum
--- PASS: TestSum (0.00s)
PASS
coverage: 50.0% of statements
ok      github.com/abcdef/example/foo     1.018s  coverage: 50.0% of statements
?       github.com/abcdef/example/qux    [no test files]

Does anyone know why this is happening and whether there's a way I can make it generate a percentage coverage across all files, including any untested files? Otherwise the coverage percentage is completely misleading.