Having a custom block in the inventory on an entity, and then fetching the inventory of said entity, the custom blocks have a "tile." prefix on their identifier.

Reproduction:

1. Download and install addon attached
2. /give @s test
3. Place the block down
4. Look at the JSON string outputted, the first index (the hand) will have the "tile." prefix on the identifier

All code is present in the server script.