When you place an armor stand on weak blocks like tallgrass, double tallgrass, seagrass, and almost every other block that usually gets replaced if a block gets placed on it, the armor stand replaces it behaving like a block. In the attachments, there is a video that shows what is wrong and another one that shows how armor stands should normally behave.

Steps to Reproduce:

1. Place tall grass
2. Place an armor stand on the tall grass

Observed Results:
The armor stand replaces the tall grass.

Expected Results:
The armor stand should not replace the tall grass.