The saltmix can be the cause when the new one has a different alkalinity reading than the old one.
Mix a small batch and check the alk and ph. Would also be good if you can test for other parameters. Tapos ikumpara mo sa lumang salt mo at sa tank water mo.
But before you conclude the saltmix is the reason, it's always good to check your tank first.
Did you add anything? Media, new sand or equipment?
Did you change anything in your regular husbandry or did you dose anything?
Anyway FWIW, BT is fine to use, regular or sps version man. Like any other salt, dapat talaga gradual ang pagpapalit ng saltmix.
BTW, speaking of mixing salts, dati yung friend ko gamit tropic marin na regular at reef version. Pinaghahalo rin nya, no problems at all.
Ako naman dati gumamit ng BT SPS bago dumating yung reef crystals ko, wala rin namang significant effect since gradual din ang change.