EVENT_COMPANION_SKILL_RANK_UPDATE will sometimes report a skill line update that is false. It happens when swapping companions. It doesn't always happen. When it bugs it happens like this:
1) I have Bastion up. He is sword/shield with heavy armor (all fully trained). He has never used a bow or worn medium armor.
2) I swap to Mirri. She is trained in bow and medium armor (all fully trained). She is wearing all medium armor and holding a bow.
3) Killing stuff (Mirri still up) causes EVENT_COMPANION_SKILL_RANK_UPDATE to trigger and report either sword/shield rank up or heavy armor rank up. More kills can cause further rank up to be reported. However when checking Mirri she still has no knowledge or advancement of sword/shield or heavy armor whichever one is being reported.
Starting with Mirri (bow/medium) and swapping to Bastion (shield/heavy) can cause reports of bow or medium armor rank up. Again; it isn't really happening.
I tried using GetCompanionSkillLineDynamicInfo(skillLineId) to filter out the false information. However; when this happens it reports the same false rank values.