Ringtone cutting off after a few seconds

This might have been an issue for me for a while, but have only recently become fully aware of it. When my phone rings, only the first few seconds play, then the phone goes silent. This happens regardless of the "Attention Aware Features" being enabled or disabled in the "Settings > Face ID & Passcode" section.

Going to "Settings > Sounds & Haptics", under Ringer and Alerts, moving the slider causes the ringtone to start playing, but stops after about 1.5 seconds. Change with buttons makes no difference. Under Sounds and Vibration Patterns, I select Ringtone. I have made some custom ringtones in addition to the built in ones that iOS provides. No matter which one I choose. the ringtone will play in its entirety. However, when I go back to "Sounds and Haptics", and repeat the aforementioned step, the same cutoff occurs.

Interestingly, it is only the ringtone portion that is affected. Other notification sounds such as messaging, email, etc work just fine.

Running iOS 11.4. I have rebooted my iPhone, and it does not make a difference. Switching from silent mode to normal mode does not make a difference either. Can someone help me please?