This may not be usual bug but... I played a game with 10 colonists with all skills at 0 (Prepare carefoully on, but it havent impact on this) and a person that was set to craft wasn't learning crafting by making bricks or smelting metal. Only by tailoring. I saw that bug when she had crafting at lvl 3. I only made 10 jackets before but she shouldn't have three levels upgrade by making those. She was making bricks all time and not learning crafting. The same happened with every colonist on a draft. They weren't learning shooting (this one could be programmed like this so it isnt the main problem).