Depending on what your MipMap setting is, you can't see chunk borders in certain view options. You can also only see chunk borders when looking directly forward, despite pressing F3+G. If you set mipmap to various levels, you can only see it in certain points of view.

E.g. Mipmap = 3, Can only see in second person.

Mipmap = 4/2/1, can only see in first person.