The resource pack attached to MC-237955 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.

The bug

The back faces of spawner blocks do not render. This results in spawners looking rather odd, as only the faces closest to the player are visible, making it look like the block only has one to three faces total.

This can be considered a parity issue, as spawners in Bedrock Edition are not subject to this.

How to reproduce

1. Obtain a spawner via commands (the Creative inventory will not work for this, see MC-132820):

   /give @s minecraft:spawner

2. Place down a single, isolated spawner block
3. Observe how many faces are visible. Depending on the view direction, between one and three faces are able to be seen.
4. Now go around to the other side of the spawner block
5. Note how there are indeed faces there as well, despite the original view direction implying there are no faces with spawner bars.

Expected result

These back faces of the spawner would be visible from all faces.

Actual result

The spawner block just doesn't show the back faces, which looks very strange.

How to fix

The resource pack attached to MC-237955 fixes this issue completely. A new "hollow cube" template model is created by it, based on the old powder snow model. Powder snow, spawners and both types of azalea leaves (MC-171859) are all pointed to this model, resulting in them having internal faces and fixing this issue.