Yep: there's a system>privacy>permission manager on the phone which lists all the permission types
However the only ones that the EonNext app appear in are camera; location and notifications, these are currently set to "not allowed" which is what I selected, but they do have a toggle to allow or not.
In the old phone the storage app permission is actually a toggle in the Android permissions (which has 3 toggles for Camera, Location & Storage)
But I know that Google have become very restrictive with storage permissions with the latest version of its OS...
So the Storage permission on my V13 phone only has a couple apps listed, with a sub-section called "all file access" which restricts access to very specific apps such as file-managers and other storage orientated apps.
I sure this must be the restriction causing my issue - I've had experience with this with another app where the new security regulations meant if you only have the phone's internal onboard memory, they could only write to their own internal folders in their app's specific /storage/emulated/0/Android/data/...{app-folders}...
I have a feeling that if you do have an SD card mounted "externally" there isn't a problem with writing to this, but think that when I installed my SD card I chose the option to use it to extend the internal-memory, so as far as my phone is concerned it's all internal memory, not a separate "external" SD card.
I'm sure someone with a bit more knowledge of Android app-development etc will be able to throw more light on this