This happens very rarely to me, I thought it was a graphical bug of the game but looking at this thread, maybe not. It is very inconsistent in my experience, here are some examples of when they happened for me:

-happens sometimes if I am on a really awkward angle like the camera limit being near a wall
-happens sometimes when I'm looking down on my character and I pass by a higher roof
-when my camera coincides with the angle of sunlight (mainly sunsets)

This triangles/glitches vary in color for me, sometimes blue, sometimes orange and will flicker rapidly for 1-2 seconds.

Only happens in FFXIV though for some reason, and not within the plethora of games I play.