This is certainly puzzling, and I have been experiencing it on a much smaller scale. I've had this happen to a couple of corals, but I'll usually see some of my best growth right before they go down hill.
Questions:
Dumb question, but are you shaking the Alk container to mix it up before dosing?
Have you recently started mixing new water from a fresh salt bucket?
Are you testing the Ca and Alk levels of your freshly mixed water before water changes?
Another way to approach the issue is what corals in your tank are doing very well? Is anything responding very well to the environment that might give clues as to what's going on?
I would consider testing your Alk 2x a day for the next few days to see if there are significant intraday and day to day swings. As others have suggested, adding your Alk part daily in small doses is beneficial. But that wouldn't explain why your tanks was doing so well... then stopped doing well.
I noticed that two encrusting monti (purple haze and poker star), and my green slimer bleached. I also had a portion of my birds nest die off. I don't know what one thing may have lead to all of this, but here are observations of what was going on at the time, and maybe something on the list helps:
-Two weeks before the bleaching I changed my 12 month old geisman actinic blue with a new ati blue+.
-One week before the bleaching my MP40 became dislodged while I was out of town and the turnover in the tank was reduced to about 25x for 3 days. Softies responded favorably, LPS seemed unaffected.
-The week that I noticed the bleaching the NO3 in my system "spiked". It's been consistently undetectable with my salifert kit for ages. Then all of a sudden it was .05. 10% water change and replacing 6 week old ROX carbon with a fresh cup of rinsed carbon reduced No3 to undetectable.
-This is the week after. My Alk reading is "low". It's always between 9.0-9.3. Yesterday it was 7.4. I noticed that the line coming from my dosing pump was clogged. I also noticed that a lot of the soda ash in my Alk part had settled at the bottom of the two part vessel. I keep shaking it, but it just settles again. I suspect that it is resulting in inconsistent dosing, so I will be testing 2x a day for the next few days..