Cesium only requests level-19 tiles for some spots when zoomed in enough. Reason?

1. A concise explanation of the problem you're experiencing.

2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.

3. Context. Why do you need to do this? We might know a better way to accomplish your goal.

4. The Cesium version you're using, your operating system and browser.

Can you post a Sandcastle example of what you’re experiencing, and what locations you’re experiencing this issue in? Are you talking about using Cesium World Terrain,or your own custom terrain served from ion? Or is it served from a different source?

Cesium World Terrain’s coverage varies across the globe, so some locations may have higher resolution tiles than in others.

Thanks for the response.
It might not be very easy for me to setup a Sandcastle case, but from your questions, could it be quite possible that the cause might be from the Terrain?

It’s hard to tell without more information. Are you using your own custom terrain?