The lock is based on speed and flaps position when on ground and most probably other stuff that I forgot. It unlocks when gear up is selected. It does not locks during extension. I do not recall if I saw that into the design documents or if it was in the AMM.
Second, the TSP is not fool proof. If you determined to tail strike you will. If you have access to a full flight you could plot the column input, elevator deflection, tail clearance and in the time history you will see that PFC will modify column response to elevator when getting close to tail strike.
It does not locks on go around.