Personally, I don’t use Media Browser, so I can not be sure.
Have you followed suggestions contained in this post:
sandra142sandra louisAug '17
To make pictures permanently available in the media browser, then follow these steps:
Select File > New Album… from the Photos menu bar. Give the new album a name.
Click on the pictures you want to make available in the Media Browser.
Click the Add button.
Click the Albums tab at the top of Photos and find the new album you created.
Control-click (or right-click) the album and select Download Originals to this Mac.
By doing this full resolution version of the pictures will get downloaded that you have selected to your Mac, making them permanently available in the media browser.
Other than that,