In my experience many montipora can be a bit more sensitive and quicker to bleach than a lot of my hardier acropora. I have lost a few from low nutrients but they looked great with considerably higher levels than you posted.
I would recommend double checking your parameters with different test kits. I have found my Hanna alk checker reads a full 1 dkh higher than my Salifert. It doesn’t really mater enough to me which is correct I just aim for the middle and keep it at 9 with the Hanna and 8 with the Salifert.
Salinity is another one that’s easy to be off on. I recommend picking up the Tropic Marine high precision hydrometer to double check your refractometer readings. I was running a bit high using calibration solutions.
Also maybe you’re low on a trace element. I had pretty good success with adding some All-for-Reef into my dosing regime. I still dose standard two part along with magnesium but I worked in the All-for-Reef to add a bit of trace elements and my tanks really doing nicely.
Try a real hardy sps like purple “Milka” stylophora
Green Bali slimmer acropora, Hawkin’s A. echinata or orange M. digitata.