I hate to revive this particular thread, but wanted to note that the bug is fixed since recent update.
Now if I am watching Channel 123 via the 30-min buffer, and a recording for Channel 123 is requested, a spare tuner is now selected for that recording - presumably an affect of dynamic tuner allocation.
That is not as good as how Series 2 did it, but its preferable to losing the live buffer. I would still prefer that it just begin recording at the point in the buffer that is requested and let me seamlessly run into the recorded content. The fact that it uses a "spare" tuner, messes up my 4-way toggle of fave channels to surf in delay mode. Now 2 of my 4 tuners are duplicating the same channel and I lose one of my Faves.
But as I said, its preferable to losing the live buffer into thin air so I guess I'm thankful for that.