JDK-8352251 has the comment:

static inline RegisterMap::WalkContinuation walk_continuation(JavaThread* jt) {
  // NOTE: WalkContinuation::skip, because of interactions with ZGC relocation
  // and load barriers. This code is run while generating stack traces for
  // the ZPage allocation event, even when ZGC is relocating objects.
  // When ZGC is relocating, it is forbidden to run code that performs
  // load barriers. With WalkContinuation::include, we visit heap stack
  // chunks and could be using load barriers.
  //
  // NOTE: Shenandoah GC also seems to require this check - actual details as to why
  // is unknown but to be filled in by others.
  return ((UseZGC || UseShenandoahGC) && !StackWatermarkSet::processing_started(jt))
    ? RegisterMap::WalkContinuation::skip
    : RegisterMap::WalkContinuation::include;
}

We need to fill it out for Shenandoah.