Bottom of iron bars Z fight when placed on top of another block, and also Z fight on the top when a block is placed above it and when viewing from underneath.
Vanilla Tweaks has a texture pack under the fixes section which fixes this issue by moving the top and bottom faces of iron bars away from the block a bit, so that they don't touch and cause Z fighting. https://vanillatweaks.net/picker/resource-packs/
I am unable to reproduce this, please provide a screenshot of it occurring with your F3 debug screen enabled.