Strange behavior when i read and write a file

Hi,

I use Xamarin Forms with VS2017 and i have a strange behavior when i debug my application on my mobile phone by USB cable.
At loading, i verify if a file is update (state 1). If not case, i updating it (state 2). If i stop and launch my application again, my file come back to previously state (state 1). It's like the file update had not working. By accident, i discovered that i had to systematically modify the android manifest after a code modification (simply remove and put back any property value) else i come back to first state (state 1) of my file.

However i have "WRITE_EXTERNAL_STORAGE" and "READ_EXTERNAL_STORAGE" permissions in the android manifest and i have not exception.

I don't understand this behavior. It's seems at a mechanism of sand box by version !?

Help me to understand.

Thanks to you