Gen.suchThat not respected by shrinking after Gen.map

My team has a library which contains a bunch of predefined generators. Here's a simple one:

lazy val genNonEmptyAlphaStr: Gen[String] = Gen.nonEmptyListOf(Gen.alphaChar).map(_.mkString)

Unfortunately when I go to use it in practice, this doesn't get respected at all. (We use specs2 along with ScalaCheck, so bear with me.)

import org.scalacheck.Prop._
import org.specs2.{ScalaCheck, SpecificationLike}
import com.paypal.cascade.common.tests.scalacheck._

class ShrinkTestSpecs
  extends SpecificationLike
  with ScalaCheck { def is = s2"""
  lalal ${test}
  """

  def fail = throw new RuntimeException("aaa!")

  def test = {
    forAll(genNonEmptyAlphaStr) { s =>
      fail
      true must beTrue
    }
  }
}

This results in some output like

Testing started at 1:37 PM ...

A counter-example is '': java.lang.RuntimeException: aaa! (after 0 try)
java.lang.RuntimeException: aaa!

If I use forAllNoShrink, or if I write genNonEmptyAlphaStr.suchThat(_.size > 0), then there are no problems and I get single-letter strings. If I use Gen.nonEmptyListOf(Gen.alphaChar) (in other words, I don't map on it,) then I also have no problems. It's only after I .map(_.mkString) that it will shrink to the empty string.

tl;dr Gen.map invalidates suchThat.

Start by reproducing the shrinking behavior with Gen.nonEmptyListOf(Gen.alphaChar).map(_.mkString), comparing it with the unmapped generator and with suchThat. Inspect the Gen.map, suchThat, and shrinking behavior in ScalaCheck; done means mapped generators no longer shrink to values that violate the original constraint, while the existing examples continue to pass.