My best guess would be that the first time you set up a custom ring tone when the options panel opened you selected something other than the system default sound picker and set it to always instead of one time.
It's easy enough to reset that as long as you know which app it's using to select the ring tones.
On mine I have 3 options for picking the ring tones, solid explorer, Google drive and the default sound picker.
If I choose Google drive and select always and later want to change it I would open system settings/apps/Google drive and scroll down to set as default option and open it. On the settings menu that opens clear the defaults.
Next time you set up a custom ring tone the option menu will open again. There is a good way to use the default ringtone picker and have your personal media files and tones added to the list.
Let me know if you have any questions about this ok.