Um, this thread
https://www.cyclechat.net/threads/street-art.221565/
shows 17 photos in the first post.
The first post in the thread in the thread editing suite (actually, there IS only one post there) shows the same photos OK in the text, but below that it shows that about 90 images were uploaded to it.
It's possible that is something to do with it? Probably not, as it's in a different part of the forum. I thought there was a limit to the number of pics per thread (some I've done limited me to 10, but that was on the walking forum and Shaun may not have applied the same settings there.
Sorry, can't help, a matter for Shaun!