Light blocks have transparent textures, but don't allow transparency

The textures or light blocks as item (the ligh bulbs with numbers) have semi-transparent pixels, which become more solid the higher the light level, and smoothing out the numbers' borders. However, in-game the textures render as solid pixels only. This also causes the pixels' sides to sometimes be missing (MC-221611).
In Bedrock Edition, the transparency is correctly rendered.

The attached images show how the texture is in the files VS how it renders in game. (upscaled to 160px for better visibility)