OCIの課金額が1円になってた
Analyze Costsから確認
ブートボリュームバックアップの数が無料枠（５つまで）を超えていた
なぜ今まで気づかなかったのか？
Importantの部分を見落としていた
For paid or free trial accounts that are eligible for Always Free resources, the same 200 GB of Always Free volume resources and five Always Free volume backup resources are available. When one of your Always Free resources is deleted, some of your Always Free resource capacity becomes available. When you have available Always Free capacity, Block Volume automatically tries to transition an existing paid resource to an Always Free resource. For example, if you have four 50 GB Always Free volumes and one 50 GB paid volume, and you delete one of the Always Free volumes, the service converts the paid volume to an Always Free volume.
逆になぜ今まで課金されなかったのか？
これまで、奇跡的に無料と有料枠のバランスがうまく噛みあってた？
なんかのタイミングで崩れたっぽい
対策