"match may not be exhaustive" on JsLookupResult

I’m in the process of upgrading from Scala 2.13.3 to 2.13.4. The most conspicuous change there is the tightening of exhaustivity checks, and most of those are straightforward. But I’m a bit stumped by this one.

I have a couple of blocks of code that are roughly:

json \ "thingy" match {
  case JsDefined(tpe) => tpe match {
  case JsUndefined() => ... error value here

The Scala compiler is kicking out “match may not be exhaustive. It would fail on the following input: JsUndefined()”. Which is odd on the face of it, and I’m puzzled – there’s nothing obviously wrong with the definition of JsUndefined.unapply(). (It’s a little odd that it takes Object instead of JsLookupResult, but it looks like it should work.)

I found this on play-json 2.9.2, but it still seems to be true on 2.10.0-RC5.

Any thoughts? I can attach @unchecked to the match if need be, but the mystery is rankling.