I tried setting a unique vibration pattern for a specific thread (I texted myself)—and it worked really well. I then deleted the thread. Upon texting myself again, I discovered that it had "remembered" the previous customization I'd set.

On the one hand, this is a good feature. But, on the other hand, if you frequently change default settings that can be overridden by customization, and you customize multiple threads, managing everything could become difficult. At some point, you'll have to simply "select" every thread and customize them all at once—negating the value of a default setting.

To get around this, there should be a "reset" or "use default" option in each thread's customization menu …