If the pH is low then the TA could very possibly be 0 ppm and the indicator will turn red immediately. Trichlor is acidic and eats up TA over time and causes the pH to crash.
TA is the measure of bicarbonates in the water.
When the pH is very low the bicarbonates have been converted into carbonic acid and therefore not measurable by the TA test. Effectively, there is no carbonate alkalinity in the water since it is all in acid form.