Hey Qr! I've had a much closer look at the mesh to see what it's doing, and ive discovered the root problem. It was not decomposePar at all. The U field anomaly comes from a point pressure defect set by pRef Earlier when I was setting up the case for this boiling pressure vessel, interCondensatingEvaporatingFoam was complaining to me that it couldnt find a reference cell for p:

"[8] --> FOAM FATAL IO ERROR: (openfoam-2512)
[8] Unable to set reference cell for field p
    Please supply either pRefCell or pRefPoint"

Which I believe is likely because I had set all boundaries to have fixedFluxPressure in an attempt to make it act like a closed vessel. SO to solve this I had set pRefPoint (0 0 0.04); pRefValue 5000; in fvSolution which I assume sets the gauge pressure for the vessel, and it did stop the machine from complaining. But ive noticed that the pressure point anomaly is exact pRefCell and has an unchanging value of pRefValue throughout the simulation, so as time moves on and the evaporation raises the prtessure of the vessel, but prefcell remains constant creating a pressure field forcing U to point to the fault cell. The Faulty pRefCell can be seen in the following figure which shows a cross sectional pressure field with a low pressure single cell (set by pRef). Im still not sure why this problem is occuring, as I thought pRef just set the gauge pressure and I cannot seem to make the simulation go without it, but im currently looking more into wether interCondensatingEVapoartingFoam can at all be used in this way, given both fluids are incompressible and Im trying to change densities with phase change.... https://cdn.discordapp.com/attachments/400697259819270145/1514777533420146808/Screenshot_from_2026-06-11_14-55-17.png?ex=6a2c9a03&is=6a2b4883&hm=665d76bce316d158d3823aece70257e6f5e6030fe64c2065a7980f4087ec8ae7&