SP online - Permissions files created using a flow

I have a flow running between 2 sharepoint online sites :

"when file created or modified" on site A + condition "filename contains x" => "create file" with same filename + same content on site B

Basically I wanted a flow that would copy new and modified files from SP online site A to SP online site B depending on a trigger in the filename.

The flow is working fine.

 Now onto permissions and groups

Site A

Using default permissions groups owners and members

Site B

Using default permissions groups owners, members and visitors

The owners and members of both sites are the same. Visitors on site B have no rights on site A.

Permission inheritance was set to default "inherit from parent"

Now I added users to the visitors group (site B) and I find that some users can access (ie read) cerrtain files on site B but not all.  For certain files they are redirected to the "you need permission to access this item" page and the notification I receive (as site owner) points to the file they want but on site A!  

I sat by a visitor user going through the motion of asking for access to a file on site B (file that was created by the flow from site A).  There was no mistaking, he was asking for access to the copied file on site B, and yet the access request notification I received had the correct file (same name) but on site A.

All files were created with the same flow,  with the same destination library/folder.

I don't understand what's happening.  Are files created on site B (via flow) inheriting the permissions of site A?  

What can I do to ensure users of the visitors group to site B have access to all the files without having to request access to individual files.  I don't want to set unique permissions as  they are just too much trouble for the number of files and visitors I have.