ore_spawn_data.json gets rewritten every time I launch Minecraft. I changed the density of kaolinite from 70 to 71 just to test it and then launched Minecraft, When the main screen shows, before loading a save, I opened the ore_spawn_data.json and the value was back to 70.

Forge: 14.23.5.2854
TFC: 1.5.2.152
Other mods: no

In the config:

# If true, TFC will override default ore gen file. Pack Makers: Disable this to keep your ore gen file from being reset to default.
B:forceDefaultOreGenFile=true