Swimming against iron bars and other blocks like glass panes, stained glass panes, fences, fence gates, slabs (when you're swimming on top of them), and even normal blocks causes chunks to not render in F5 mode.
If your Z coordinate (Facing West) is less then 50, Chunks will render randomly, but if is equal or more than 50, Chunks will not render apart from the Chunk you're currently in.
This sounds like it might be MC-63070. Does that seem accurate to you?