so I got an email from support after submitting my file saying that it was the following CSS that I had in my external CSS file which was clashing with the scene thumb generation somehow:
background: url(bg.jpg) center;
background-size: 100% 100%;
It so happens that, yesterday, we decided we didn’t want to set the background image this way and had already removed that code. I didn’t notice the connection at the time but, yes, the scene thumbnails are back without that code in the CSS.
Apparently, a bug report has been filed for this issue.