I guess this could be a weird one, but in v12 Azure CDN was only working when image URLs were NOT in a permanent (/getmedia/xxxxx) format. And it wasn't mentioned anywhere on the documentation. There was a setting "Use permanent URLs" here.
Now, this setting was removed in v13 and you cannot switch back to folder URLs, I don't think so. But the article regarding Azure CDN setup was copied over from the previous version. Could you reach out to Kentico support, please? Hopefully, they will be able to advice on how to set up this correctly, if it is possible at all.