Not able to Add Any Custom Ringtone
Using ringtones on the Moto G Power: You can use default notification tones or explore other options to personalize your MultiTimer application.. In case the alerts don't work, please email us at email@example.com, so we can look into your issue and assist you.
Set ringtone for app notifications
When you are unable to add custom ringtone on Motorola Moto G-family devices
This problem has been tracked since 2018 and is not app related, but system related.
The problem occurs after updates are installed - the phones revert to the default ringtone.
You can try some checkpoints here. Hopefully this will help:
Delete the original 'Ringtones' folder, manually created a new folder of the same name and then pasted a copy of the .mp3 into that directory.
Do the same with the 'Notifications' folder.
Go into safe mode and then reboot your device.