The resource pack attached to MC-249094 offers a complete fix for this issue. Mojang have my full permission to (and are encouraged to) replace the affected vanilla model files with the contents of the resource pack.

Discovered while testing MC-249094.

The bug

The bottom of sculk shriekers, as well as the four sides, are not culled by solid blocks which are directly against them. This is due to these five faces missing cullface arguments for those faces.

How to reproduce

1. Place a sculk shrieker on the ground
2. Place four opaque blocks on each side
3. View the occluded sides in Spectator mode

Expected results

These faces would not render.

Actual results

They do.

How to fix

The resource pack attached to MC-249094 fixes this issue and that issue as well as offering code cleanup for readability and a reduced model file size, so I'd recommend using that model file to fix this issue rather than attempting to do so manually.