I've messed around a lot with this particular page, trying different Photo Grids, editing rows etc. etc. I'm more likely to believe I broke it myself rather than rubber stamping 'Bug' on the behaviour, but I thought it interesting enough to post.

Several of the grids had the same problem, but not all.

https://auzlink.net.au/Xara%20Photo%20Grid.mp4

Hmmm. I think I need to look up how to add a short video.