I would question the accuracy of your mag test results. Mag test kits can be hard to read. When calcification occurs (corals forming their hard skeletons and coralline algae growing) a set ratio of calcium, alk and mag is used. For approximately 18ppm of calcium used, 1meq/l (2.8 dKH) of alk is used and 2ppm of mag is used. So taking that into consideration, the drop you see in mag is way off. I would run a few more tests and see what kind of results you get.
On a side note, I'm not a fan of Reef Crystals, even though many people do like it. The reasons being it contains vitamins, but no one knows what they are, if they are needed and what they break down into. The parameters are also very much different (higher) than the average parameters of seawater out on the reefs. On average, Reef Crystals gives you around 490 calcium, 13 dKH alkalinity and 1440 mag. Natural seawater is around 420 calcium, 7 dKH alkalinity and 1280 mag. I use Instant Ocean, which mixes to around 400 calcium, 11 dKH alkalinity and 1350 mag.