When typing in code, code completion is triggered automatically when typing. This is because non-comment/non-string tokens have a standard type of "Other" and the Quick Suggestions setting is "On" for "Other".

However, in YAML, typing bare keys at the top level are categorised as "String" so you don't get any completion unless you manually invoke it:

But some kinds of invalid syntax are set as "Other", which means code completion can automatically trigger in the case of invalid code:

It seems odd to have code completion automatically trigger for invalid code, but not for typing keys where it's more likely to provide some useful suggestions. I'm pretty sure it never used to work like this, I've had a few reports recently that this "stopped working".