Bosses are not affected by increased respawn or the rapid respawn event. They were only affected for the first 2 rapid respawns events then it was adjusted because it made it too easy to camp them for loot.
Here is the official news when it was changed. https://www.tibia.com/news/?subtopic=newsarchive&id=4686"As a result of player feedback and to alleviate the inflationary emergence of rare boss items drops, all bosses and minibosses will be excluded from this and future Rapid Respawn events. This includes bosses like Zugurosh, Jaul or the new falcon knight bosses."
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