Lake Chad in the Beatus Map

Whilst perusing Jean Maley's "Histoire lac Tchad durant le dernier millénaire" online recently, we noticed that the author proposed an early map depicting Lake Chad from medieval Europe. The Beatus map follows the conventions of medieval European cartography inspired by Ptolemy, but does depict a large lake south of the Sahara (Deserta arenosa, or sandy desert?) in what would roughly be the Lake Chad region. The connection to the Nile does match the incorrect view of Islamic geographers that Lake Chad and the Nile were connected. It probably resulted from a mistaken belief that the Bahr el Ghazal and the Nile were one and the same. But is it possible this map only placed Lake Chad in its correct location through chance or error? It's clearly based on ancient maps since it still refers to the "Isle of Meroe" and does not name specific places in the Lake Chad Basin. Yet unlike Isidore of Seville, the large lake believed to be the source of the Nile is not named Nilides, nor does this match show the river turning south before rising north to Egypt. 

Fra Mauro and Lake Chad

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Fra Mauro's depiction of the Lake Chad region correctly named some of the peoples and features of the area, like Bagirmi, Bulala (after they were the dominant power in Kanem), and the Mandara Mountains. He must've had 'native' informants from Kanem or Borno or access to decent Arabic sources by geographers.

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But why continue to use the name "Organa" instead of Kanem? Fra Mauro knew of the Marghi, Mandara, Bagirmi, Bolala (Bulala in Kanem and Lake Fitri), and perhaps he thought Lake Chad was a giant marsh due to its seasonal fluctuations. But why continue to use the name Organa when his Arabic and/or Africans sources have specified Kanem and Borno as place names by the mid-15th century. Was it due to the fact that the Bulana rulers had already seized power in Kanem by this time? It could have been due to the already established pattern in medieval European Cartography to refer to Kanem as Organa, something which was already apparent in the 14th century map of Angelino Dulcert.

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The Angelino Dulcert map also mentions the king of Organa fighting naked "blacks" by the sea, which I interpret to mean Lake Chad. There's a French translation and commentary of his map here that makes it even more likely to be Kanem.