Uups, sorry. Problem solved.
Missing "NodeBB is rebuilding front-end assets" was the problem.
I upgraded our Dev environment to 1.17.0 this morning and any topics that had a thumbnail before the upgrade no longer display one.
The environment was upgraded from 1.16.2 to 1.17.0 and when a topic is edited the thumbnail still displays in composer but it does not display on the topics list page.
If a new topic is created it does display the thumbnail as usual though.
Our forum use-case relies on thumbnails a good deal so this is definitely a problem for us.
@julian It doesn't show up in any of those views: category, recent or unread.
@jminer Thanks, I'll take a look. I had added in backwards compatibility but seems like it isn't working 100%.