AHHHH HAAA!!!!
We've figured it out......or at least close.
You gave me the info i need. ALTHOUGH boiled water gets rid of chlorine....you are also getting rid of the purest water. It goes up in steam. Essentially making your leftover water's, parts per milllion count go up.
What you've done is make your water HARDER. The concentration of calcium will be higher in boiled water.
Excess calcium will lockout MG. I know this because I did the exact same thing. Started boiling my water for my 3rd grow....and my plants did what yours are doing.
You can change this very quickly by watering with Reverse Osmosis water, or plain bottled water will do. Tap water that's left to sit open for 24 hours NOT BOILED. With your next watering.
It's prolly not that you need to ADD Mg, it's prolly already there, but the roots won't take it up.
If you think adding is needed, you can use Molasses, 1tblspn per 2 L of water.
Molasses has usable Mg among other things that are good for the plant.