Well, ive seen it across three operating systems (Linux, windows, android). So if you've seen it too on Mac/safari, its safe to say its probably a site specific integration. Hopefully unlock origin adds a blocker for it (though definitely not guaranteed as its not technically an ad). In the meantime, might be worth adding a specific block for the pop up element in UBO. Though not sure if the latest safari has UBO support so I don't think that helps you.
What browser are you using? I've seen it too. At first I thought it was a new Firefox feature but I've seen it on fennec and the desktop Firefox. I'm thinking its a back end integration with specific sites. Hopefully its not Firefox.
This is one of the reasons I do intermittent fasting. I basically hold off on eating anything until around 4pm when I have a snack, skipping breakfast and lunch. I either have coffee or black tea once or twice but am careful not too have too much as it can make me sick on an empty stomach.
So, if it works for you, I recommend giving IF a try. Its pretty hard for some people and easy for others, but if it helps it might be worth it.
Alternatively, experiment with your carb / sugar intake. Try eating protein / lean meals for lunch and see if that helps.