Weird scoverage issue

build.sbt

// Tried with 2.11.8, 2.12.3, 2.12.4
scalaVersion in ThisBuild := "2.12.3"

// Tried with scoverage 1.5.1, 1.6.0-M3
// Tried with sbt 1.0.4, 1.1.1

lazy val commonSettings = Seq(
  parallelExecution in Test := false,
  organization := "com.example.blah"
)

lazy val coverageSettings = Seq(
  coverageEnabled := true,
  coverageHighlighting := true
)

lazy val userApi = (project in file("user/user-api"))
  .settings(commonSettings)
  .settings(coverageSettings)
  .settings(
    libraryDependencies ++= Dependencies.userApi
  )

lazy val `blah` = (project in file("."))
  .settings(commonSettings)
  .aggregate(userApi)