Hello Muted_messages.
Welcome to Apple Support Communities. We understand that you hide the alerts for a group message thread but it won’t remain on after enabling alerts for the thread again. We’re happy to help.
Are you connected to Wi-Fi or cellular data? If Wi-Fi, we recommend testing on another Wi-Fi network. If you haven’t tested, we want to see if this works as expected using cellular data. This will allow us to isolate the cause further.
We recommend using the steps here to back up and update your iOS version: Update your iPhone, iPad, or iPod touch - Apple Support
Please see the section titled “Manage notifications” here: Use notifications on your iPhone, iPad, and iPod touch - Apple Support
Here’s additional information on using messages here: Use Messages on your iPhone, iPad, or iPod touch - Apple Support
Let us know if you have further questions.
Take care.