Another behavior I've noticed is even though the REST API call executes, it is not always logged regardless of the status code (200, 201, 400, etc).  Specifically I noticed this when GETing and POSTing to Pages DB.

GET could be because it was returned from the CDN cache rather than from the server so it won’t be logged at all.

POST should def. Be logged if the endpoint is configured to be logged. 
I’ll take a look at pages endpoints next week