Expectation:

render distance should be roughly spherical around the player, no preference for x, y, or z axis. 

Actual Behavior:

chunks on the x-axis (east/west) of the map are hidden by some kind of culling or other rendering behavior. The effect is that the rendered area around the player is like a strip that is longer along the z-axis than the x-axis. I use the word culled to describe the invisible chunks on the x-axis because they become visible immediately after walking toward them along the x-axis, pixel by pixel. I estimate the distance to this culling wall is about 16 chunks as it does not appear at a render distance setting below that. 

steps to reproduce:

On my iPhone 11, open a new flat creative world. Set render distance to max (36). Observe the world from above. Fly along the x-axis and notice how it comes into view different from unloaded chunks.