The Bug:
Normaly, if you place an armor stand in the grass, it will breake, so the armor stand can be placed, but this time, when you place an armor stand in the grass, the grass will not breake, and the armor stand will get trought it
How to reproduce:
1.- Take an armor stand
2.- Search for a site with grass or tall grass
3.- Place the armor stand on the grass or tall grass
4.- You will notice that it doesn't breake
Note: It also happens with 1-block flowers and saplings
Note2: You can place the flower/sapling/grass and then the armor stand and still happens
This is working as intended; armor stands are entities, not blocks, so they can be placed 'inside' blocks where player can stand too (like torches and flowers)