Allright I ran some tests
All worlds were generated in a 0% below scorching temperature PW map unless stated otherwise.
All worlds use the same elevation, temperature, volcanism and savagery map unless otherwise specified.
Elevation map contains a west ocean and east mountain range.
Rainfall shadow map generated at 321.1 degrees.
Results:
World 1.
Rainfall: 0% below desert, 0% below grassland, 100% below forest.
Drainage: 0% below Dune/swamp, 0% below Rocky/Grass, 0% below Hills.
World was entirely tropical except for a few regions which had a rainfall value > 80
World 2.
Rainfall: 0% below desert, 0% below grassland, 100% below forest.
Drainage: 0% below Dune/swamp, 0% below Rocky/Grass, 100% below Hills.
World was entirely tropical except for a few regions which had a rainfall value > 80
World 3.
Rainfall: 0% below desert, 0% below grassland, 100% below forest.
Drainage: 0% below Dune/swamp, 100% below Rocky/Grass, 100% below Hills.
World was entirely tropical except for a few regions which had a rainfall value > 80
World 4.
Rainfall: 0% below desert, 0% below grassland, 100% below forest.
Drainage: 100% below Dune/swamp, 100% below Rocky/Grass, 100% below Hills.
World was entirely tropical except for a few regions which had a rainfall value > 80
Special notes: Abundance of Mangroove swamps. Appear to replace tropical saltwater swamps where drainage < 9. Further testing required.
World 5.
Rainfall: 0% below desert, 0% below grassland, 0% below forest.
Drainage: 0% below Dune/swamp, 0% below Rocky/Grass, 0% below Hills.
World entirely temperate.
World 6.
Rainfall: 0% below desert, 0% below grassland, 0% below forest.
Drainage: 0% below Dune/swamp, 0% below Rocky/Grass, 100% below Hills.
World entirely Temperate.
World 7.
Rainfall: 0% below desert, 0% below grassland, 0% below forest.
Drainage: 0% below Dune/swamp, 100% below Rocky/Grass, 100% below Hills.
World entirely temperate.
World 8.
Rainfall: 0% below desert, 0% below grassland, 0% below forest.
Drainage: 100% below Dune/swamp, 100% below Rocky/Grass, 100% below Hills.
World entirely temperate.
Special note: No mangroove swamps, instead temperate saltwater swamps. Confirms the suspicion that mangrooves replace only tropical saltwater swamps with a drainage < 9.
Further testing for mangrooves:
World 9.
Rainfall: 0% below desert, 0% below grassland, 100% below forest.
Drainage: 100% below Dune/swamp, 100% below Rocky/Grass, 100% below Hills.
Temperature map 100% below hot.
No mangrooves appeared, world was entirely temperate.
World 10.
Rainfall: 0% below desert, 0% below grassland, 100% below forest.
Drainage: 100% below Dune/swamp, 100% below Rocky/Grass, 100% below Hills.
Temperature map 100% below scorching.
Mangrooves appear.
Special note: nearly no tropical climates. Tropical climates occured in regions with temperature values > 105. Mangroove swamps occured only in regions with a temperature > 105.
Conclusion: Drainage appears unrelated to the problem. Rainfall appears to be the only correlation to the occurrence of the problem.
Mangrooves swamp generation requires: Rainfall value 66-80 (66 because that the lowest value for swamps, and 80 cause over 80 they turn into temperate climates.), Drainage < 9, Temperature > 105
I hope this gives enough proof of the problem, it would be nice if someone else could confirm this for me. If it's a problem solely I am having there might be something wrond with my world gen, though I have not modified these files whatsoever.
EDIT:
I was wondering whether a temporary solution to this problem could be minimising the maximum rainfall value to 80 and making the threshold for forests contain the rainfal values of 75-80 or 70-80 directly controlling the amount of broadleaf and conifer forests as well as swamps, circumventing the tropical to temperate issue?