Boof helpfully highlighted that some of my stadiums had the background image split in different sections - below is an example from the thread:
I had adion take a look and posted on the forum, and was given an elegantly simple solution. You can find the details
in this post in the bug thread, but the short version is models need to be a certain size in the game for them to read correctly. The easy way to fix this is to add a triangle further back in the model:
And then your backgrounds should load correctly:
