The bug

Forest and taiga type biomes spawn with large treeless areas when ore vein spawn sizes are set to excessive values.

How to reproduce

1. Create a new customized world and edit the Andesite and Diorite spawn sizes to 100 in the Presets screen. I tested on the seed -802523552 in versions 1.9.4 and 1.12.1.
   Used preset:

   {"coordinateScale":171.103,"heightScale":684.412,"lowerLimitScale":512.0,"upperLimitScale":512.0,"depthNoiseScaleX":200.0,"depthNoiseScaleZ":200.0,"depthNoiseScaleExponent":0.5,"mainNoiseScaleX":80.0,"mainNoiseScaleY":480.0,"mainNoiseScaleZ":80.0,"baseSize":8.0,"stretchY":12.0,"biomeDepthWeight":0.0,"biomeDepthOffset":0.0,"biomeScaleWeight":0.0,"biomeScaleOffset":1.0,"seaLevel":79,"useCaves":true,"useDungeons":true,"dungeonChance":21,"useStrongholds":true,"useVillages":true,"useMineShafts":true,"useTemples":true,"useMonuments":true,"useMansions":true,"useRavines":true,"useWaterLakes":true,"waterLakeChance":4,"useLavaLakes":true,"lavaLakeChance":80,"useLavaOceans":false,"fixedBiome":-1,"biomeSize":6,"riverSize":30,"dirtSize":33,"dirtCount":0,"dirtMinHeight":0,"dirtMaxHeight":256,"gravelSize":33,"gravelCount":0,"gravelMinHeight":0,"gravelMaxHeight":256,"graniteSize":50,"graniteCount":2,"graniteMinHeight":0,"graniteMaxHeight":32,"dioriteSize":125,"dioriteCount":1,"dioriteMinHeight":32,"dioriteMaxHeight":96,"andesiteSize":50,"andesiteCount":4,"andesiteMinHeight":96,"andesiteMaxHeight":192,"coalSize":17,"coalCount":20,"coalMinHeight":48,"coalMaxHeight":192,"ironSize":9,"ironCount":20,"ironMinHeight":41,"ironMaxHeight":96,"goldSize":9,"goldCount":2,"goldMinHeight":41,"goldMaxHeight":48,"redstoneSize":8,"redstoneCount":8,"redstoneMinHeight":0,"redstoneMaxHeight":25,"diamondSize":8,"diamondCount":1,"diamondMinHeight":0,"diamondMaxHeight":16,"lapisSize":7,"lapisCount":1,"lapisCenterHeight":25,"lapisSpread":16}
   

2. Fly around the forest and observe there are fewer trees than there should be