Inconsistent position (degrees) after warping

I’m trying to figure out how to use the warper object and I’m currently facing an issue running this piece of code:

from psychopy import visual, monitors, event
from psychopy.visual.windowwarp import Warper

warp = True

mon = monitors.Monitor('tryMonitor')

stimWin = visual.Window(
    [1920,1200],screen = 0,fullscr = True,
    monitor = mon, units = 'degFlat',
    useFBO = True,
    allowStencil = True,

monitorWidthDeg = 104.8

gratingBack = visual.GratingStim(
    stimWin,tex = 'sqr', mask = None, units = 'degFlat',
    maskParams = None, pos = (0.0, 0.0), 
    size = [monitorWidthDeg, 10], 
    sf = 1,
    ori = 0.0, phase = (0.0, 0.0), texRes = 128,contrast = 0.9,
    opacity = 1.0,autoDraw = False

if warp:
    warper = Warper(
        warp = 'spherical',
        warpGridsize = 300,
        eyepoint = [0.5, 0.5]
    warper.dist_cm = 20

keepGoing = True

while keepGoing:
    if len(event.getKeys(keyList=('escape','q'))) > 0:
        keepGoing = False

I know my monitor width in degrees, and I can confirm it because, by not using the warper, I generate a stimulus like this one:

Moreover, i know the width is correct because if I reduce the size of the grating to 99% the original size, I get this stimulus:

However, when applying the warper, the size of the warped stimulus is extended beyond the limits of my screen. Now I need to downscale the size of the grating to 80% to be able to see the entire stimulus:

This raises a problem: If you specify the position or the size of a stimulus in degrees, its actual position on the monitor is not physically accurate anymore after the warping. Am I missing something or is this the expected behavior?

is it correct to set the units to degFlat in this instance?