Normally, colder water has a higher pH naturally (i.e. cooling pool water without doing anything else to it will raise the pH by about 0.2 going from 85F to 50F) but your lowering of TA and targetting a pH of 7.5 has made your water slightly corrosive, but nothing to worry about (index is -0.3) especially in cold temps where reactions are slow. So relax and just let the pH drift up naturally if it wants to do that, but if it doesn't, that's fine too.
Though I target around 7.5 for my pool in the summer, it natrually drifts to 7.7 in the winter as the temperature cools down. Then when the water warms up again, it naturally comes back down to 7.5.
Richard
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