What might cause this? I have optifine installed, but I wasn't sure if there was a specific setting that causes this that I might be able to turn off.

was this ever figured out? im having troubles with it and cant find any sourses to help. Optifine doesnt effect it and i dont have shaders or anything like that on, so im not sure how to fix it

The outline disappears when I look at it from the other side.

Without optifine at certain distances from the pennants the black box still appears. No difference between internal shaders and no shaders selected.