Apparently, since some kind of update (Python? Pillow? GTK/GDK?), supported images fail to be loaded properly. Instead, you get the icon used in case of failure.
Not sure how to replicate this reliably but using a "big" image cache and multiple extraction threads on a directory with lots of images that are difficult to load and decompress should help. In my case, the second page (using single-page layout) is often affected but in can affect any image. Jumping far and quickly between pages might also help, resulting in many cache misses.
Forcing a reload proves that the affected images can in fact be loaded and displayed just fine.
My current hypothesis is that there might be some internal changes that now require proper synchronization and/or flushing, something that used to be unnecessary. Might be directly related to [a4092eb4f2927ce5d600444d0b10c308148ca34d].
Update: Further observation suggests that only AVIF files are affected. Thus, it might be an issue with a third-party lib. Using Arch Linux here, not sure which package might be responsible. I tried experimenting a bit but could not yet reach a conclusive answer. Any help is appreciated.