It's been more than a year since you posted this, and perhaps you solved it. But perhaps also not, as I think I had exactly the same issue. I've been looking for a solution for months, searched the Internet for many hours and spent a lot of the time with the always very friendly, but not very helpful Samsung support staff (sorry).
I was experiencing the following. In apps such as Outlook, AirBnB, Evernote and a few more, I would only see the albums camera, screenshots and downloads, when I wanted to attach or use a picture. So, not all albums I had were visible in the so-called Media Picker (in album view). Only with many additional clicks was I able to get to the Gallery app, which worked as a sort of (inefficient) work around. Not very helpful at all when you have organised everything neatly in folders. Especially not if you are not looking for the most recent picture, and you have hundreds or even thousands.
The solution consists of a few steps, which could vary depending on how you use your phone. Sounds simple, well in fact it's pretty crazy in my opinion.
1. First of all, you must have Google Photo's installed. Perhaps you thought, just like me, the Samsung Gallery app works fine, let's get rid of all this garbage on my phone. Nope, it must be installed.
2. You must enable Google Drive back-up in the settings of the Google Photo's app. Hey, wasn't there this great integration with OneDrive? Well yes, but this must be turned on nevertheless.
3. Then you must move the albums that you were missing or didn't see in your apps. Go to a file explorer application and move the desired albums from the folder DCIM to the folder pictures. Yes, you heard that right. Don't worry however, the album will still be visible afterwards in your Samsung Gallery app (should you still want to use it after this).
4. Now, as a final step, head back to the Google Photo's app, and search for the option where you can select which folders to back-up to Google Drive. Select the folders you need to see in the Media Picker when using apps such as Outlook and possibly many others.
5. Now, go back to the app you had the problems with and you will see the folders you didn't see before, when trying to attach or use a picture in that application. One way to check the correct setting (beforehand) is to view the section "On this device" in the Google Photo's app. If the album (created in the Samsung Gallery app) isn't there, it won't be visible in several or many other apps. If the album you need to see is not there, check the execution of steps 1-4.
Albums created directly in the Google Photo's app do not have to be backed up to Google Drive to be visible in the Media Picker. However, you also won't see them in the Samsung Gallery app. So, make your choice how you want to work.
Good luck.
