has_dependent_backup of backup is false, but the backup has a child backup that its status is available

After backup A created a snapshot, backup A created a error incremental backup C, has_dependent_backup of backup A is false at this time. Backup A created a available incremental backup B with snapshot, two incremental backups were created based on backup A and has_dependent_backup of backup A is true at this time. After error incremental backup C deleted, has_dependent_backup of backup A become false, but backup has a dependency backup B that its status is available.

After error incremental backup C deleted, backup A and available incremental backup B as follows:

+-----------------------+--------------------------------------------+
| Property | Value |
+-----------------------+--------------------------------------------+
| availability_zone | nova1 |
| container | 77/b0/77b05a77-5f1a-49b1-8764-e13ed10822d7 |
| created_at | 2021-01-14T01:41:29.000000 |
| data_timestamp | 2021-01-14T01:41:29.000000 |
| description | None |
| fail_reason | None |
| has_dependent_backups | False |
| id | 77b05a77-5f1a-49b1-8764-e13ed10822d7 |
| is_incremental | False |
| name | fjkA |
| object_count | 214 |
| size | 10 |
| snapshot_id | None |
| status | available |
| updated_at | 2021-01-14T01:50:17.000000 |
| volume_id | b6e4b83e-3025-4f28-9370-417d784379a8 |
+-----------------------+--------------------------------------------+

+-----------------------+--------------------------------------------+
| Property | Value |
+-----------------------+--------------------------------------------+
| availability_zone | nova1 |
| container | a2/31/a23155f3-7ec3-4fcc-80b3-bc8be47926ef |
| created_at | 2021-01-14T01:47:28.000000 |
| data_timestamp | 2021-01-14T01:44:18.000000 |
| description | None |
| fail_reason | None |
| has_dependent_backups | False |
| id | a23155f3-7ec3-4fcc-80b3-bc8be47926ef |
| is_incremental | True |
| name | fjkB |
| object_count | 0 |
| size | 10 |
| snapshot_id | 87b032ec-8171-450b-ba9f-a7f85312b2d6 |
| status | available |
| updated_at | 2021-01-14T01:48:24.000000 |
| volume_id | b6e4b83e-3025-4f28-9370-417d784379a8 |
+-----------------------+--------------------------------------------+

Try to delete backup A, as follows:

openstack volume backup delete 77b05a77-5f1a-49b1-8764-e13ed10822d7
Failed to delete backup with name or ID '77b05a77-5f1a-49b1-8764-e13ed10822d7': Invalid backup: Incremental backups exist for this backup. (HTTP 400) (Request-ID: req-c900063c-3e12-4951-86d6-4b451deac808)
1 of 1 backups failed to delete.