Python 3.7.3
opencv-python 4.1.0.25 from pip install opencv-python
I can open and write webp format from file with no problem, but I can't decode from memory. The code (at the end) returns:

imdecode_(''): can't read data: OpenCV(4.1.0) /io/opencv/modules/imgcodecs/src/grfmt_webp.cpp:164: error: (-215:Assertion failed) data.rows == 1 in function 'readData'
However, it works with opencv 3.4.2 from conda install opencv.
Also, Kaggle kernel environment (Python 3.6.6 and opencv-python 4.1.0.25) just returns None object without throwing an error.

img = cv2.imread(file)
retval, buf = cv2.imencode(".webp", img, [cv2.IMWRITE_WEBP_QUALITY, 100])
img = cv2.imdecode(buf, 1)