Thanks for the report @luo. I can reproduce the results on my machine.
I think it has to do with an object-pooling PR that made it into the 1.0.0 release: Add object pooling by ZackOfAllTrades · Pull Request #244 · CesiumGS/cesium-unity · GitHub. If I undo the changes made by that PR, the memory will reset every time it enters Play Mode. The mesh pool is expected to expand when you load more meshes, but it seems to happen even if I don’t move the camera around much. We’ll have to look into this to figure out what’s going on.