I have a similar issue but don't know if it's related to the number of shows. Some days ago I added some new shows and all at once the icons for the first four shows in the listing were gone. One of them was newly added so my first thought was "damn, maybe something broken with this show". Updating the shows didn't bring them icons back.

Today there was an update of seriesfinale and after updating and starting I gave itr another try and updated all shows. After a while it finished and all icons were back! But sadly they are gone again after a restart of the application

Don't know what to do right now. Maybe jrocha could give some hints on how to preserve the icons?