The bug

Spawner blocks do not cull the faces of other spawner blocks directly adjacent to them.

Such culling is generally characteristic of other transparent blocks, such as glass and slime blocks (MC-119856). (Leaves not doing this is very likely a stylistic decision to make trees appear more realistic.)

How to reproduce

1. Place a spawner
2. Place another spawner directly next to it

Expected result

The face between the spawner block would not be rendered; this is the behaviour seen for glass.

Actual results

It is.