Gap Issue on EPSG:3857 quantized-mesh

Hello,

I'm working on my own implementation of the quantized-mesh which use the EPSG:3857 projection, a valid format according to the quantized-mesh specifications. I provide my quantized-mesh through CesiumTerrainProvider.

When I'm using my custom quantized-mesh, holes appear on the map after a specific zoom. This issue appeared in all projections (2D, Columbus View, 3D).

I realized that it had something to do with the "available" section in layer.json. I had 14 lines in my "available" section and gaps would appear in 3D projection after zoom 14. You can find the layer.json file attached (as .txt).
I added the missing lines in the "available" section and that solved my gap issue in the 3D projection... but not in the other projections.

Since the 3D projection is working I suppose that my quantized-mesh implementation is fine but that there is an issue with EPSG:3857 since it ain't the default projection. Can you tell me if you think that the issue comes from my quantized-mesh implementation, or is it a known issue? Since I don't own a server to serve the tile I can't replicate it in a Sandcastle.

[layer.txt](https://github.com/CesiumGS/cesium/files/7122885/layer.txt)

Start with CesiumTerrainProvider and the attached layer.txt, checking how its available section and EPSG:3857 terrain data are interpreted across 2D, Columbus View, and 3D. A reproducible served quantized-mesh dataset is needed; done means identifying whether the gaps are caused by Cesium or the supplied quantized-mesh data and covering the confirmed behavior with a regression test.

Written by the indexing model from the issue text.