Before reaching T15, defeating the map boss does not drop the map.

during my endgame progression to tier 15, I often find that I defeat map bosses but no map drops. I don't know if this is a mechanic or a bug.

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BlackCat#7088 wrote:

during my endgame progression to tier 15, I often find that I defeat map bosses but no map drops. I don't know if this is a mechanic or a bug.

Assuming this is for PoE 1 (where you posted), map bosses are not guaranteed to drop maps. There are some passives to boost the chance that the final map boss drop a map, but it's not guaranteed by any means and you'll want to acquire maps from various sources.

Other sources include other league mechanics, Kirac (missions and shop), stacking more mobs in a map such as with shrines, or buy any missing maps from other players if you're in trade.