While I understand you may not like that behavior, It isn’t a bug and is intentional. That is not to say it may not change some day.
The watchlist is saved remotely at plex.tv for your account. We intentionally don’t want to know what is on your server and don’t know things like the non default or custom movie posters you may have chosen. That is why we call it the Universal Watchilst in docs Universal Watchlist (Beta) | Plex Support
Things added to the watchlist can be available from multiple sources. A user might have added a thing from their server, from friend server, from Discover, or when watching a trailer in Plex for something that is not available to stream anywhere yet.
When you click on an item in the watchlist and go to the details page it is a Universal Detail page, not the details page exclusively from your server which the item might not even be available on. That universal details page then loads all the sources that item ID is available for the account that is signed in in the “Watch from these locations” row next to the poster.
