Confirmed and fixed. You were on one of the quests that never got wired.

The Alphabet Soup Pasta event ships as FOUR separate quests -- two MapleStory ones and two Verse8 ones. Only two of the four had drop rows, so whichever of the other two you took, not a single letter could ever drop. Your list of ten (M A P L E S T O R Y) is exactly the set that had no source.

All four quests now drop their own letters off any monster while the quest is active. Nice catch: this one was invisible from our side because the event 'worked' on the two quests we happened to test.

Fixed on develop, not yet in the published build.