[Kameron8]

I found a way to consistently generate dense tiles with a different set of abilities -- hopefully it lends some insight into why this ancient bug is happening. If somebody is boxed in by dense objects (people, Yokai, destructible rocks, etc), and they are forcibly pushed but cannot move anywhere, they will generate dense tiles in all corners around them.

This is illustrated here, where a single player is bashed while surrounded by 4 units:



The dense tiles persist past the battle, lingering in all tested battles on the same map (in this case, battles in the Shrine) until whenever the map is unloaded in the game. In our instance, it took between 2-10 minutes for the dense tiles to stop showing up in battles in the Shrine. This does not explain why some abilities seem to randomly generate them outside of this circumstance, like Piercing Shinken from the original post, but it's possible they're related.

So far, this has been reproduced with Impact Call pushing a boxed in monster, and the bash example above. Both result in identical spawn positions of the dense tiles, so I'm lead to believe any knockback suffices in this reproduction.