iPhone 15 iMessage Alerts Not Working When Phone is Locked

I am currently using an iPhone 15 on with Straight Talk on Verizon towers. After updating to iOS 27 earlier in the week, I noticed that visual and auditory alerts for Messages were working intermittently, even though notifications for every other app were working perfectly. The texts and red notification circle on the app icon was there and displayed the correct number. The issue persists whether the sender is using iMessage, RCS, or regular SMS. Through troubleshooting and experimenting, I have narrowed it down to iMessage. If my iMessage is on and my screen is locked, I get no auditory or visual text alert. Any other scenario and I receive alerts as expected.

I have gone through and made sure of the following:

• Focus is off
• Silence is off
• Volume is up
• Notifications are on, including for the lock screen

Troubleshooting steps I have taken:

• Toggled all Message notification related settings off and back on, both individually and simultaneously
• Tested multiple notification sounds, some custom and some stock
• Turned iMessage off and on
• Turned text message forwarding off and on
• Updated to iOS 27.0.1
• Reset all settings
• Restored to factory settings

Unfortunately, the issue persists. What other suggestions are out there?