Description:
If an armor stand with the "Marker" attribute is given motion via the motion tag, it will interact with the next block it comes into contact with as expected, then ignore all other blocks until it falls through the world into the void and despawns.

Expected behavior:
opaque blocks prevent entity from moving through the area contained within that block

Observed behavior:
entity is able to move through an opaque block

Reproduction:
1) Facing north toward any wall, execute the following command:

/summon minecraft:armor_stand ~ ~ ~ {Marker:1,Motion:[0.0d,0.0d,-1.0d]}

2) Observe that the armor stand travels north, bounces off the wall, then falls through any opaque blocks into the void