8 cups of MA in 18,000 gallons will lower the TA by 14 ppm and your pH would have been lowered below 7.4 but with carbon dioxide outgassing (even faster with aeration) would have the pH not drop as far. 6 pounds of baking soda in 18,000 gallons would raise the TA by 24 ppm. So the net of the two should only have raised the TA by 10 ppm which with test error might not be measurable. Remember that MA lowers TA.
The combination of MA and baking soda lowers pH with no change in TA if you use balanced amounts of the two. In your 18,000 gallon pool, if you add 5-3/4 cups of full-strength Muriatic Acid (31.45% Hydrochloric Acid) then that lowers the TA by 10 ppm. If you add 2-1/2 pounds (about 4 cups) of baking soda, then that raises the TA by 10 ppm. So if you added both one after the other, the TA won't change but the pH would drop. How much the pH drops depends on the TA level (and whether you have borates in the pool) but with 50 ppm TA the pH would drop from 8.6 to 7.3 if there were no carbon dioxide outgassing.
