I just had the exact same issue, my calcium and Mg jumped from 440 to 750 and 1340 to 1100 in 3-4 days according to the trident. I got the replace reagent 2 notification so I went ahead and did both the CA and MG, and now they test normal again. Didn’t notice anything weird and there was a good amount of reagent still left in both bottles.
I did some research when this first happened to me and found that the angle of the intake tube in the bottles can cause this, one of their solutions was to overfill the bottles so having extra reagent when it’s time to change them is normal. I’ve also found that they can drift off if the Trident isn’t calibrated every two months.