I have found that alk drops around 0.5 dKH every couple of days. This is with regular Instant Ocean stirred with a powerhead. I mix larger batches than I need for a single WC and so I dose alk to match my tank params before I do the WC. I would suspect other trace elements precipitate as well. Probably why folks recommend using ASAP.
I've had bad luck on dosing the same day as water changes with Red Sea Pro running so high on the elements. I dose the day after WC.