The problem arises form the parameters used in detectMultiScale() method, try different parameters for optimal results

import numpy as np
import cv2
from matplotlib import pyplot as plt

bodydetection = cv2.CascadeClassifier('cascades/haarcascade_fullbody.xml')
img = cv2.imread('/content/full-body-shot-of-young-blonde-happy-woman-isolated-on-white-background-ER3EPG.jpg')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
body = bodydetection.detectMultiScale(gray, 1.01, 2) # Further adjusted parameters
for (x,y,w,h) in body:
   cv2.rectangle(img,(x,y),(x+w,y+h),(0,255,0),2)

cv2_imshow(img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Below is an example of my input image