Based on the shared screenshot provided, I looked at the CORS Policy and found out that the most secure solution to ensure images are properly displayed is - as described on the Capture signatures and drawings article - will be to find a different way to provide the image.
One alternative is to upload the image through the app itself using an Image column, then use the value that is written into the sheet as the initial value of the Drawing column.
Note that images hosted on Google Drive using the export link format do not work as background images due to CORS limitations.
This was happening to me a month ago and it was solved via a support ticket but since about a week ago it started happening again. Images are showing as text.
I will contact to support about this issue again. And please Google Dev, Do some testing before deployment some update,This is become very annoying problem that it’s broke again.