When I wrote the post on reusable blocks in a Custom Post type, I thought I had everything working. Unfortunately, I discovered a bug!
While the reusable block seemed to load and work perfectly, once the post was saved and reloaded, an error occurred:
The work around is to immediately edit and save the block before the post is saved. I do not know why this works or why the issue occurs to begin with. I have filed a Gutenberg issue for this and will update once I have an answer.