The new render optimization introduced couple versions back seems to work only at special angles. For example, if I look north (the direction at which the optimization seems to work appears to change randomly), the current number of rendered chunks is 1 (C:1/3600), if I move more than 45 degrees of north or closer/further from the point this happens, it jumps back up to ~200 (Render distance is 7, all scenarios tested underground). This also causes edges of the screen to become black causing an "X-ray" like effect through which particles from torches can be seen.

This also seems to effect rendering of underground caverns and lava lakes - if I load up a resource pack which has all blocks transparent except bedrock and lava, I can still see random patches of the underground render which disappear and reappear when I move around.

This causes the game to drop in frames per second as it basically renders more than whats on screen (including random parts of the underground).

Attached below is an example.

I will/can attach both server/client configurations if further needed.