This can happen both with native monsters and e.g. sausage goblins. The drop line is absent from the combat log and the drop is not acknowledged. A sample debug HTML where the "3 piles of Elf Army machine parts" drop was not parsed:
Right, so this looks like the message ""Arr," says a nearby Crimbuccaneer. "10 points for ye, to be sure."" is being interpreted as damage. After that, all messages between that and the stats get skipped.
Not sure if this's the proper topic, but it seems like we can get 3 pieces of 12, but the monster info says 1-2. This is separate from the additional piece from the map. Meaning that you can get 4 pieces per fight, rather than just 3, if a map is active.
1-2 is the base, plus +0-1 from the accessory plus +0-1 from the consumable effect (this goes into the main line) plus 1 from a map active (this goes into a separate line that mentions the map), up to 5 total.