It wouldn't surprise if the problem was wit the salt mix. I use Instant Ocean Reef Crystals. BRS had to replace a 200 gallon box of the stuff I purchased when my all was only mixing in the 4-5 range. The new batch mixes ALK to about 8.
But now the Mag is off the chart.
There have been other posts about Instant Ocean salt mix not mixing to the indicated numbers on the box.
I feel there is a QC issue with this brand as of late.
Are you using standard Instant Ocean or are you using Reef Crystals?
I have been using reef crystals. I just tested a sample of mixed water and it took 11 drops... 196.9 ppm the hydrometer read 1.024