This bug has been around since 1.16 and has went unresolved since.

Back in 1.14 custom blocks that were created by addons now had the abililty to show up in the creative inventory and recipe book. But then when 1.16 came around it bugged out and it doesn't work anymore.

How to Reproduce:

Create a custom block in a addon and load the world in creative, you'll see that the block does not appear.