Minecraft: 1.20.1
Forge: 47.2.20
Quark: 4.0-436
Tested on a minimal Quark instance.

When lava and water meet at a spot where a horizontal iron rod is placed, then the iron rod gets replaced by cobblestone. This does not happen with vertical iron rods, specifically only horizontal ones.

It's not the cobblestone that breaks the rod, it's the fluids, horizontal placed rods break when any fluid touch them (or at least lava and water)