Nodes sometimes disappear from the repository, but are actually still in the database

I have the following problem:

On our homepage there is an image. This image is uploaded by a cms user. When the image was first created all image related nodes where send to all nodes in the site cluster. The image is shown in all frontend nodes.

Now the next morning the image was gone on one of the site servers. The site servers have an embedded repository. After some research the 'picture' variant of the image does not exist (anymore?) , but the thumbnail is still there. This only counts for one of the site nodes in the site cluster (6 nodes in total).

After a restart of the site (and repository) this issue is gone. Could it be related to some sort of cache, which is not invalidated properly?