Lava doesn't render under copper grates properly

In the game, non-flowing lava (source lava block) is not visibly rendered under copper grate blocks if there are any flowing lava that's coming from the source.

Steps to reproduce:

1. Open Minecraft in a creative (or survival) world
2. Place non-flowing lava
3. Place copper grate on top of the lava

Expected results:

The lava is correctly rendered.

Observed result:

The lava is not visibly rendered under the copper grates.

Information:

This seems to be fixed on 1.21.20.21 PREVIEW version.