Android increasingly does not allow even apps with full storage permissions to actually read any file from storage they did not make themselves. If you run into this problem, there are two possible workarounds:
Enable Manage All Files
This is a special permission some apps like file managers can declare. Cheogram Android optionally declares this (unless you get it from Google Play) so you can enable it in your OS settings and then the app can actually see all files.
Re-parent the files you wish the app to have access to
adb shell content update --bind owner_package_name:s:com.cheogram.android --uri content://media/external/images/media --where "relative_path=\'Pictures/Cheogram/\'" adb shell content update --bind owner_package_name:s:com.cheogram.android --uri content://media/external/video/media --where "relative_path=\'Movies/Cheogram/\'" adb shell content update --bind owner_package_name:s:com.cheogram.android --uri content://media/external/audio/media --where "relative_path=\'Download/Cheogram/\'" adb shell content update --bind owner_package_name:s:com.cheogram.android --uri content://media/external/downloads --where "relative_path=\'Download/Cheogram/\'" adb shell content update --bind owner_package_name:s:com.cheogram.android --uri content://media/external/file --where "relative_path=\'Documents/Cheogram/\'" adb shell content update --bind owner_package_name:s:com.cheogram.android --uri content://media/external/file --where "relative_path=\'Movies/Cheogram/\'"
Note for Google Play users it is com.cheogram.android.playstore