I doubt that there would be a problem with WordPress 5. While the changes within WordPress were dramatic, this functionality would not be related to that.
If it generates an error when trying to download the file saying that the file doesn't exist, to me that would indicate that there is some write permission and the file is not being created corretly.
I know that when I drop to command line in my WP site I can't find the file because it's some WP magic, but I can get the file downloaded fine.
Maybe @rovidx_media has some idea for your issue.
To be honest, I have also not been able to get my WP-Smart TV to work either, but I am able to download the file. My created json just flat generates an error on Roku DP setup, saying that the images are wrong and that the feed doesn't exist, when I hand create the JSON it works just fine with the exact same links so I am still working on what the heck is going on with mine. ( http:/domainname.com/wp-content/uploads/2018/12/mountains-1920x1080-800x450.jpg ) is how the image links show up while it should be (
http://domainname.com/wp-content/upload ... 00x450.jpg )