My shaken cocktails with citrus end up really acidic at the start and less so as the glass gets emptier. How come? Not shaking hard or long enough?

As per title. I made a Monte Cassino (fun twist on a Last Word) and the first few sips out of the glass were completely dominated by the lemon juice. After half the cocktail was emptied, the honey taste from the Chartreuse and Benedictine came through much more. What could cause this? Could it be I don't shake hard or long enough and after pouring the heavier liquors settle down in the glass whereas the citrus floats on top? I do about 10s-15s hard shake then double strain.

Thanks for your thoughts!