As above.

Here's the context where I ran into this issue:


(Those're ultra dense steam ducts)

Verifying with a minimal setup in creative, it seems to happen with all duct types, and connect to all fence-type blocks.

Bottom to top the duct types are balefire plasma, kerosene, petroleum gas, and ultra dense steam
Left to right the blocks are Dark Steel Bars (Ender IO), Iron Bars, Steel Fences (Immersive Engineering), Oak Fences, and Nether Brick Fences

Fixed it - will be in next update