Onyx Boox Poke 3 screensaver not updating

I have KOReader writing the current book cover to a file.

(The path is /storage/emulated/0/Pictures if you're interested; I'm saving it as a .PNG).

The inbuilt Gallery app shows the latest cover in the Pictures folder. So it's saving correctly.

The screensaver app shows a book I read previously, though, not the new cover. It finds the correct filename, though, but just shows outdated content for it. There's a button at the bottom of the screensaver app that doesn't do anything other than make the screen refresh.

The screensaver itself doesn't update either; it shows a book I have finished reading, not the current contents of the PNG file it's pointed to.

Is there anything I can do to make it update? How do I stop the cover image being cached somehow?

Otherwise it's a great device, and KOReader is an excellent e-reader - but the cover image thing is a bit frustrating...