(Also link following ...)
This doesn't look like your first rodeo per se, but perhaps some good vibes and ponderings will help.
Curious as well what led you to your original declaration on the MG def as noted in the first post and what overall change you applied to correct. Single dose correction or increase in your CalMG up to the current amount? What you did there in way of amend or what not is probably inline with what is happening now.
Some damage will show up later on from those original problems from when they are younger after you flip. Sort of strange but the hit for trying to divert into flowering will again zap and test your soil health/feeding schedule.
Eating Mg like scrogy said is more or less expected. It's one of the curses of RO IMO. RO is great, but so many formulas seem to almost be made for harder water. Hence calmag is always part of your add unless you're getting it from a nute line in heavier doses. Showing up now when you may have only barely corrected your feeding schedule originally would be inline for sure.
Gaging soil feeds off of PPM can be a little off as well. Base charge of some soil mixes (whether intentional or stated correctly), water temp, and all that jazz can really impact resultant PH and absorption rate. Yes, water temps dont matter in some places. Much debate. But when making solutions from nutrient slurries, uneven mixing can occur below some thresholds, etc.
Without being able to stick my fingers in the soil and all that though, I get rather despondent about pics and "remote viewing" (pardon the tinfoil hat implieds) when things like this come up. At least for my old bones, nothing is the same until I can taste and touch it.
Having played with the toys, I found most loose soils that are not over-watered or drench, seem to support the 1 EC range. Or more or less hitting up to 1000 PPM when graduated into it. Most feeds over that amount in smaller containers (5 gallon and less) just lend toward burn or waste. Conditions and strain will flux that to some degree, but that's my take on what to push for. Using bottle amounts for PPM measure doesn't seem to line up most of the time either. But that level of voodoo when it comes to soil just doesn't help me much.
Right, not sure any of that helped, but that's partly why I lurk. Too hard to not ramble or go all OCD with the basic questions.
Cheers.