I believe (but not 100% sure) they were all displaying correctly once, but then stopped for some reason.
One thing for sure is that they are stored correctly in the Coda server, indeed you can download them and they would open up. They are not corrupted files.
We had a couple other bugs pop up around images and how they displayed in various browsers. I think this is one that was fixed when the other bugs were being worked on. I don’t doubt you saw an issue, it just happened to show itself and then get fixed in a small timeframe.