Chris,
The artifacts that you see are due to the interaction of the horizontal surface pressure features with the vertical interpolation. Take a look at the following presentation, slides 4 and 5:
https://www2.mmm.ucar.edu/wrf/users/tutorial/202001/wang_runtime_options.pdf
The problem is due to the horizontal discontinuity of the vertical indexes of neighboring grid points when using different vertical levels (from the first guess data) for the same k-index for the WRF computational surfaces (usually the first few levels near the surface). This occurs when the WRF topography (and therefore the WRF surface pressure) is changing in a dramatically different way than the first guess topography (the first guess surface pressure).
In banded pressure regions (such as the artifact that seems to follow a 2-4 hPa wide zone, I could not see exactly due to the image resolution), those grid cells that are all using the same first guess vertical levels for the interpolation give that resulting red-banded value for the U field.
The first guess isobaric data traditionally does not have well-matched values at the surface and at the nearby constant pressure surface as it intersects the surface. Compounding this is the influence of interpolating to a second surface (from first guess to WRF). The WRF topo can easily be 500 m above or below the first guess. When the WRF model topo is below ground, a simple assumption about a constant U V extrapolation is used. When the WRF model topo is well above the first guess surface, then free atmosphere winds are used (as opposed to something more appropriate to the surface layer). Even without a topo difference (such as over the ocean), the vertical selection of the trapping pressure levels from the first guess will give the artificial elevation shift.
This is what you are seeing. These artifacts are always most obvious over the ocean, as there are no other competing problems. There are interpolation options that are available. The good news is that the model does not need to be run, the real program can be run for a single time period - until you get the desired results. So this experimentation is relatively cheap. As you have found, another solution is to use first guess data that does not have these thick, flat input surfaces (i.e. use native model coordinate data, as opposed to the processed isobaric fields).