This is the answer.
I would never try to flush with just Magnesium myself.
Too much mag can compact your soil and cause many other issues.
Gypsum will work to reset the soil CEC, open up the soil and provide much-needed Calcium.
Now I would flush with a mix of Gypsum and a small amount of Epsom salt.
I would also ditch the bottle of Cal/Mag.
When you need Calcium you should add calcium and when you need Magnesium you should add Mag.
But adding both at the same time regardless of what is needed is not the best choice in my opinion.
When they are both in the same bottle you can not change the ratio of Cal to Mag ever, you just keep adding both at the same ratio.
Also to get the amount of calcium most plants need, you will be over applying Mag.
So in short, if you use a bottle of Cal/Mag you are most likely to have too much Mag messing things up in your soil.