If you keep the pH the same, which you did by adding Muriatic Acid along with the 20 Mule Team Borax, then the TA will not change very much at all with 50 ppm Borates. At a pH of 7.5, it only goes up 5 ppm. That is normal because most of the borax becomes boric acid which does not count towards TA. Only the smaller amount of borate ion will count towards TA.

Remember that TA is a measure of the pH buffering capacity against a drop in pH. The Borates have a greater capacity against a rise in pH. They would measure high in "Total Acidity" as opposed to "Total Alkalinity", but we don't have a pool test for that and it's not really that relevant since one measures the TA more as a measurement of the carbonate in the water so one can calculate the calcite saturation index.

You can see how the buffer strength changes at varying pH in the post pH Buffer Capacity. You can see how the carbonate buffer system gets stronger as the pH drops, while the borate buffer system gets stronger as the pH rises.