You seem to be using a not supported method of mixing which involves mixing all the nutrients in a small jar before you use them. I am not sure why you are doing this method, but its probabl the source of your problem.
Second, you are mixing 3 separate nutrients all at once, yet somehow picked Bud Explosion as the nutrient that was not dissolving fully. You probably decided this because some of the particles are white and the same color as Bud Explosion. However most likely the particles are small calcium pieces from Mega Crop (not BE or SC).
So even when doing your method, the results are somewhat predictable, however once they enter your main reservoir and mixed with water they should eventually dissolve, even if it takes 1 day. Are you still having the problem with the small white particles even after a full day in the main tank? You are right, that its not that big of a problem, it only represents like 1% of Calcium which eventually should dissolve.
Also, a second solution to your problem would be not "premixing" like you are doing. Just stir/slowly mix in the nutrients into your final reservoir and they should dissolve without having this issue.
Let us know how it goes and your progress! Also posting a picture would be useful