I already know this. However IT IS A Facebook issue that I have worked on for several months now. Even Facebook admitted it was them...but won't tell how to stop it. Did tell me one thing but it only helped for a few days.
If Facebook has access to your gallery/photos/files/media (in most cases, it does) it will show photos from your device. You can change the setting to have access only to storage/files so you may still save pics from Facebook to your phone while limiting access to your pictures.
Check your permissions for all Meta apps. There are 3 hidden system apps in addition to Facebook/messenger/Instagram. I don't use Facebook so I don't know all associated apps you may have.
