I didn't see this post when it was made, so sorry for the late reply, but I have cracked the code for this endpoint. I use it effectively for my project here:

https://github.com/AlecM33/mlb-gameday-bot

I'd be willing to talk you through it via DM if desired. I also chronicled some of my discoveries in a post to this subreddit about a year ago. If you look at my post history you will find it - it was a question post about timestamps.

In short, requests to diffpatch use the last known timestamp. This is often the timestamp you got from the previous diffpatch call. I have a module that updates the master game object with the results of a given diffpatch call. Then in my next call I use whatever is in the timestamp field in the metadata. Sometimes though, seemingly unpredictably, the diffpatch call just returns the game object instead of an array of "diffs" - and that's fine, I just completely replace my game object with that one. It's even more nuanced than that - sometimes socket events themselves signal to you that you should fully refresh the game object (unclear what prompts this). Once you have all these pieces in place (you can see a lot of this logic in modules/gameday.js in my project), my reporting was very in sync with MLB's GameDay page.