The armor stand textures are broken. It seems that the game's programming can't find its original textures, and is instead substituting whatever textures are present around it. 

You can check yourself by loading any world and placing down an armor stand.

The attached image is what occurs when there are no entities in the immediate vicinity of the armor stand. The game has created some sort of amalgamation of every texture in the game.

I haven't seen this bug before today, December 10th, 2020. The version I was playing on was 1.16.40 by the way. I hope this report helps!