[MCM-1312] The entity filter "is_underground" returns true when there is a slab or some transparent blocks above the entity

Partner Rank: 4

Partner Team Reporting: Starfish Studios

Verification builds: 1.16.210.05, build 5526115

Summary:
The entity filter "is_underground" returns true when the entity is under a slab or some non-opaque blocks.

Impact:
This bug prevents an entity that should always test if there are no solid blocks above it to work properly when it's under one of these blocks:

Slabs
Leaves
Ice
Beacon
Cobweb
Anvil
Cauldron
Hopper
Brewing Stand

These blocks were tested based on the beacon that works when under all transparent blocks, working properly under them. This is affecting a small functionality of a beacon-based entity of Starfish Studios content in review.

Repro Steps:

Download and install the attached behavior pack.
Join a world with the behavior enabled.
Run the command /function beacon_test.
Summon a sheep /summon sheep.
Place any of the listed blocks above the sheep.

Repro Rate: 3/3

Observed Results:
As noted in step 3, the beacon is still activated even though these blocks are on top of it. When there is any of the listed blocks above the sheep, it instantly becomes sheared even though these blocks are not entirely opaque. But it doesn't become sheared if below a stair (which should be similar to a slab) or below a glass (which should be similar to a beacon).

Expected Results:
The sheep should only instantly become sheared when under a full-opaque block and not become sheared when under any blocks that the beacon is able to be activated under.

Screenshots/Videos attached: No

Regression Builds: None

Notes:
The entity filter "is_underground" also strangely returns true whenever the entity is on top of Slabs, on top of a Brewing Stand or inside of Cob Webs.