I've noticed what is either a bug or my misunderstanding of the four-rock free guard zone rule. 

I led with a corner guard about as wide as the 8-foot and outside the house, but beyond the top of the house.  My opponents first stone tapped it so it was just biting the house.  My second stone was a corner guard to the opposite side.  My opponent's second stone was a full-weight take-out of that first stone that was now a "biter".

I understand the rule to be:
"Until four stones have been played (two from each side), stones in the free guard zone (those stones left in the area between the hog and tee lines, excluding the house) may not be removed by an opponent's stone."

Was my opponent able to convert my first stone into a non-free-guard rock by tapping it into the house or is this a bug that only looks at the status of a rock for the current throw?

this is real curling not all games are the same 

Yes, once a rock is in the house whether put their by you or tapped in by your opponent, it is no longer protected.

I suppose that it is a rare enough occurance that it doesn't really matter much.

I do see the occasional wildly off-weight stone, even though I thought I was using about the same timing on the mouse button.

Great game and great fun though!  I take its "features" along with the gameplay.  When I no longer like it I will vote with my feet and leave. 

I guess the most annoying one that I've come across is that the heaviest weight NEVER curls the same. I expect it to be a little inconsistent to reflect real ice conditions, but when you factor in that the big weight should barely curl at all, and sometimes in this game it curls as much as a draw, the big weight is almost completely unreliable.