My Galaxy S7 edge recently updated to Oreo. Now play store apps no longer have rights to write to my external SD card. I have followed app instructions about how to open a system folder chooser dialog, and selecting the root of the external SD Card. Unfortunately the external SD card is never an option. The upper right 3 dot menu only has the option "Show internal storage" and does not have "Show SD Card" and the upper left 3 bar "Open From" menu only has "Recent" and "Downloads."
I've also set up a FTP connection to my phone but get permission errors when trying to delete or write files to the external SD card (I can read from it just fine).
The camera and other native apps allow using the external card as the storage location and the "My Files" file manager has full access. I am not rooted.