Can't access Personal Hotspot over Bluetooth with another iPhone

I have an iPhone 4S, running iOS 5.1.1. (Though this issue has existed since 4.3)

I enable Personal Hotspot on the iPhone. Bluetooth and Wifi are turned on.

My iPad 2 (running iOS 5.1.1) sees the iPhone in the list of available Bluetooth devices, connects to the iPhone, and voila! The iPad is tethering my iPhone's internet connection. When the two disconnect from one another (like when I leave the house). I simply pick up the iPad when I return, go to Settings>General>Bluetooth and tap my iPhone in the list of available devices, and (as long as my iPhone is within range) the iPad connects, and I'm online again. Painless!

The problem arises when I try to connect my iPod touch or spare iPhone 4 to the Personal Hotspot of my iPhone 4S via bluetooth.

I enable Personal Hotspot on the iPhone 4S. Bluetooth and Wifi are turned on.

I pick up my iPod touch. Bluetooth is enabled, but the iPhone 4S does not appear in the list of available Bluetooth devices.

I pick up my spare iPhone 4. Bluetooth is enabled, but the iPhone 4S does not appear in the list of available Bluetooth devices.

The problem is, if I want this iPod touch or spare iPhone (acting as an oversized iPod touch) to access my Personal Hotspot, I have to unlock my iPhone 4S, go to Settings>Personal Hotspot and wait for the iDevices to connect; something I don't have to do with my iPad. Additionally, Personal Hotspot connections made over Wifi use far more power on both devices than Bluetooth.

The weird thing is, there was a week or two (before updating to iOS 5.0) that my iPod touch was able to see my iPhone 4, connect to it via Bluetooth, and tether off the Personal Hotspot painlessly without me ever having to unlock the iPhone, and open Settings.

To be thorough, I tried using my iPhone 4 (and even an old iPhone 3GS running 4.3) as the 'host' devices for my Personal Hotspot, and every time the iPad had no issues connecting, but all the other iPhones saw no available Bluetooth devices.

Also, there are other discussions on this issue, I've tried to 'bump' them by commenting.

I genuinely believe this to be a software flaw, but I'm open to suggestions. Thoughts?