Most likely motion smoothing not working good enough in this case. ASW2 could get better results if the game is also feeding it the depth buffer (I believe @CodeLoran69 made a topic asking for this), and it is possible it is already feeding it but I doubt it.
NB: there are similar artifact displaying with SteamVR + Index, although not looking exactly the same. However with SteamVR you can reduce and nearly eliminate these in adjusting the “prediction” slider when using fixed frame rate. This works good for me with 18fps → 90Hz + 33.3ms