Yep. An incomplete nitrogen cycle can deplete alkalinity. There's a unit of alkalinity consumed in the process of converting ammonia to nitrate (by aerobic bacteria). That unit is replaced in the process of converting the nitrate to nitrogen gas (by anaerobic bacteria). Anaerobic bacteria populations grow more slowly than aerobic bacteria, so during cycling, we often see a build up of nitrate. If we perform a water change at this stage (to reduce the nitrate), the alkalinity that was lost will never be replaced.
How are you testing salinity? Is your meter calibrated and, if so, with what? Finally, as Nikki noted in the post above, both calcium and magnesium is relevant here. But since this is relatively new saltwater, there *should* be sufficient magnesium in the mix to prevent abiotic precipitation; unless, of course, there was a quality control failure at Fritz - which isn't beyond the impossible. (Every once in a while you hear about a bad batch of salt coming from various manufacturers.) This, though, is pretty uncommon.