Custom logotype disappears

if I change the logo in my installation (through Themes - Colorscheme - adapt - Logotype) it's visible for a while, but after a while, it becomes a broken link and the URL to the file doesn't work anymore.
If I go back to the settings for the logotype, I can see the custom file as an option and if I pick the same file again it'll be available for a while until it stops working again.

It's not easy to reproduce for sure (and so debug) but here is what we know:

• whatever is the root cause the result is an inconsistency between the document in cache and what's in the database (the attachment is not deleted but it vanish from the cached document)
• in consequence a restart (or a script to invalidate the cache) is enough to fix the issue
• it's not related to filesystem attachments since it also happen with attachment stored in database
• it's not related to image cache