The standard logic for extracting "// Output:" comments in example tests is here:
https://cs.opensource.google/go/go/+/master:src/go/doc/example.go;l=113;drc=28f1bf61b7383bd4079d77090e67b3198b75be12
Observe that it has a ^ anchor so that it matches only the start of the comment text.
By contrast, pksite uses this regexp:
https://cs.opensource.google/go/x/pkgsite/+/master:internal/godoc/dochtml/internal/render/render.go;l=24;drc=7431aa5932c48bce425bf3f2dc368c5b1fed6e2c
which lacks the anchor. The consequence is that an example such as this one:

func Example() {
    // A
    // B This Example does not have an "// Output:" comment so it will not run.
    // C
}

is considered executable by pkgsite, and its rendering of the example is abruptly cut off at line C.