This has been an issue on many Galaxy products since the recent oreo update.
This has been a solution to many, including myself...
Here is the path for the reset.
Settings (widget symbol in upper right) Apps (Default apps, App permissions) Tap on the three-dot symbol on upper right of APPS menu Select Show system apps Select Media Storage Long press on Storage Press both Clear Data and Clear Cache Clear Data will give you a big warning that application data will be permanently deleted. This is not deleting your photos and videos. It is only deleting how the apps find them. Once this is deleted the apps will start finding them again which is what resolves the problem. Wait for apps to find all the photos and videos. You can use your phone during this time but it may take awhile until you see all your pictures.