Ok, I was able to reproduce this and made a PR for a fix. This issue only pops up when a user has instance blocks and works fine otherwise, which is why others in this thread (and myself when I was testing) aren't running into issues.
The root cause really is that I was creating schema for input and output validation to match the manually created swagger docs, but the output for instance_blocks was different than specified (and I marked it as a required field, likely from a copy/paste error). So, thanks for helping us as we are going through endpoints and making them self-documenting.