I would think that's not an ideal way to mix the water. You should imo mix it the entire time your making it. I would think you are almost forcing precipitation out of it by letting it sit highly concentrated in the bottom of the bucket unmixed.
I think if your mixing 20g it might be a good idea to just get a 20ish gallon container and use a powerhead the entire time so you get the proper aeration needed and a full mix. I've always thought and followed the 24hr constant aeration/mixing is needed to fully mix the salt unless using certain brands that are designed to be used instantly Like aqua vitro (above) is meant to be used in 4 hours or else it drops cal/alk out due to precipitation.(yet the directions say 24 hours) .
I had the same issue that this thread is referring to and it was from keeping aqua vitro water stored for later use I despise this salt mix lol. I wasted a good chunk of change switching to it and later having to add cal/alk everytime I wanted to do a wc.