I have completed the learning for the N1 kanji schedule for a while now. Right now I am reviewing the words and my mastery level for the words are around 5-7. Recently a few of these kanji mastery have suddenly become level 1 even though I have not been answering them incorrectly.
The words you're studying are adding new vectors. If a kanji had no 音読み・訓読み available to study, and you add one by learning a word, that drops the level of that kanji in your schedule. Mastery is global, so the mastery you're seeing in your schedules isn't the "true" mastery for those terms. Every vector has its own mastery.
For example I have 羽 at level 8, but if I check the 音読み (which I didn't have up to that point) it drops to level 1.
