Hotlinks protection was in fact turned on. I have added the subdomain to the allow list, and "all is well."
Prior to changing the hotlinks allow access list, I created a test subdomain on another website, copied the images and html, and it worked, even though the newly created subdomain was (is) not on the allow list ?
I still do not know why one site behaved differently than the other, but it's all working now and I've learned from the experience.
Thank you all for your perseverance.