From your description it sounds like the 'Files' app has just been inadvertently set up so it's tied to the incorrect Google account. The issue being that while your phone did get linked to your primary Google account whenever you first set it up way back, you can install apps that 'might' link to your primary Google account by default, most of them can be set up to link to whichever online service you prefer -- i.e. if you install an email app you have the option to tie it to a different GMail email and/or other third-party email services, along with your primary Gmail email.
If you look into the settings menu of that Files app itself, are you sure there's no option to change which Google account it links itself to? If not, go into your phone's Settings >> Apps menu, find and open the Files app entry, tap on the Force stop button, and then find and tap on the Clear data button. Using Clear data will wipe the app's setting configuration files, essentially returning the app back to its original, first-time used state. Start up the Files app and it should prompt you with the initial setup process so you can tie it to your primary Google account this time.