Since I am the Accidental TFS Administrator, and since I'm trying to move my organization to use the cloud for VCS instead of our out of support TFS, I've gotten into YAML. But like everything else where I work, they don't provide any training or support. I'm just through headfirst into the deep end of the pool.
Until I read today's editorial, Steve, I'd never even heard of a "closing key:value". After posting this, I'm going to have to look that up.
To answer your question, no I've not done anything to ensure that the YAML I'm working with is complete. Although, I'm as sure as I can be that it's complete, as much as anything else in a Git repo is complete. The YAML I work with is a part of our Git repos. So, either Git restored the whole file, or it doesn't. And that's as likely to be true of any source code file in the Git repo.