BUG ALERT IN SCRATCH 2.0 EDITOR
Scratch version 445.1 almost fixes the issue. If you create a new project using extension blocks and save the project, you can reopen the new project without issue. However, if you have a project you saved previous to version 443 and load that project, the extension blocks still are shown as undefined. This is being investigated and documented here.
Here is an example of what I mean. The lone Digital Pin block was added and saved with version 445.1.