Set forceSmartPSDPixelScaling to true if layer is a placed smart object.

Photoshop returns an error to getLayerPixmap() if the layer is a linked smart object, links to another PSD and has scaling options set. This is a workaround until the root bug is fixed in PS.
Yes, I think there is a bug in the docinfo data.  If I look at the JSON for the offending linked smart object layer I see this...

{
      "id": 7158,
      "index": 148,
      "type": "layer",
      "name": "WildLeaves-guitar",
      "bounds": {...},
      "boundsWithFX": {...},
      "layerEffects": {...},
      "smartObject": {
             "ID": "2468e79c-7c47-1177-b169-cb563f1a977d",
             "placed": "246501e8-7c47-1177-b169-cb563f1a977d",
             "antiAliasType": 16,
             "type": 2,
             "transform": [...],
             "comp": -1
       },