Texture variations on custom blocks not working

Partner Rank: Normal (3)
Partner Team Reporting: Panascais
Verification Build: 1.20.10+
Regression Builds N/A (always observed to be broken)

Summary:
When using custom blocks and defining random texture variants in the terrain_texture.json it only takes the first texture instead of using all listed textures and their weight for randomization.

Impact: Development quality increase and custom block usage limitation decrease.

Example pack attached

Reproduction steps:
1. Download example.
2. Load testpack.mcworld into your game
3. Go into creative
4. /give @s panascais:block.mailbox
5. Place custom mailbox several times

Observed results:
Mailbox texture stays blue and doesn't change

expected results:
Mailbox texture changes everytime you place it to another color (4 in total)