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:
Download and install addon attached
/give @s test
Place the block down
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.
This has been fixed in 1.12.0.9