File storage apps such as ownCloud and Nextcloud use the Android Storage Framework. Thus their data are stored inside a sub-folder underneath /ownCloud and not inside /Documents (which is not synced per se).
It should be possible to place an obsidian vault into an ownCloud sub-folder to get it synced between mobile and desktop devices. This already works fine with the Obsidian builds on Windows and Linux, but not on Android. Therefore I assume a bug in Obsidian for Android, possibly related to the usage of the native Android storage framework.
I think this has to do with the way Android works. In Android one application can access files within a container only and not all the files on the device (unless it is rooted).
My opinion here is that Obsidian should support adding a fourth option at “Open another vault” step called “Open cloud folder as vault”, which will have a sub-option to choose a local folder.
At the surface level this will work the same way it works today with folders. Behind the scene it should keep syncing with the cloud folder.
So if you are on android the “Select sub-folder” provides an option to choose a folder only inside the android/com.obsidian folder. I guess android users can settle for that much given the limitations.