min example of failed ++2.11.12 commandAlias

build.sbt

import Dependencies._
  
ThisBuild / scalaVersion     := "2.12.8"
ThisBuild / version          := "0.1.0-SNAPSHOT"
ThisBuild / organization     := "com.example"
ThisBuild / organizationName := "example"

lazy val root = (project in file("."))
  .aggregate(mA, mB)
  .settings(
    crossScalaVersions := Nil,
    name := "crossCmd",
    libraryDependencies += scalaTest % Test
  )
lazy val mA = project.settings(crossScalaVersions := Seq("2.11.12"))

lazy val mB = project.settings(crossScalaVersions := Seq("2.11.12"))

addCommandAlias("validate", "test")

validate is just an alias of test

sbt "++2.11.12 test" works as expected but
sbt "++2.11.12 validate" throws

[error] Expected configuration
[error] Expected ':'
[error] Expected key
[error] Not a valid key: validate (similar: ivyValidate, update, evicted)
[error] all mA/validate mB/validate

also sbt ++2.11.12 test (without the quote), actually will run tests on root module, which is sort of surprising, although I can understand why it was that way.