/execute detect functions inconsistently with semi- (snow layer, grass path, soul sand, farmland) blocks

When experimenting with the new command blocks I noticed that blocks that are not full or "normal" blocks function highly inconsistently when using a /execute command with the detect function inside a repeating command block or on a redstone clock.

The command I used was the following:
/execute @e[r=20] ~ ~ ~ detect ~ ~-1 ~ <block here> 0 /effect @e[r=20] slowness 1 1 1

• Stone, Sandstone, dirt, and other normal blocks work; the execute command will trigger every tick when in a repeating command block or every time redstone triggers a normal impulse block
• Grass Path blocks (/execute @e[r=20] ~ ~ ~ detect ~ ~-1 ~ grass_path 0 /effect @e[r=20] slowness 1 1 1) will trigger once when the player steps on the block, but will not trigger again until the player steps off the block, where it'll trigger again

The attached pictures show how the slowness effect remains while on a full block (stone), but do not when on a semi-block (grass paths).

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Code analysis by Marcono1234 can be found in this comment.