CalMag....yes....right from the start... My hydro guy is very strong on 50% of the main nutes being CalMag and using RO water (my hydro guy walks on water btw) then the rest in bloom nutes then supplements. It's always worked well for me.

Sounds like your ph is fine.....seriously if it's at 5.8 vs 6.0 during bloom or day 6 or whatever dont' stress.....it doesn't have to be perfect. If you have your ph between 5.6-6.0 I think you're good....that's my rule anyway and again...never had issues.

Maybe just not enough CalMag? Or perhaps too much of something else (at 1700) that would cause the plant not to uptake a certain nutrient etc....everything interacts. (I say that like I know what I'm talking about but I don't really....I read it somewhere )

Good luck