I realized that this might not be an issue with scan_consistency at all and it is such an edge case. It only happens when I flush, drop the primary index, and re-create the primary index. I really don’t expect to do that on production. Even in test environment, deleting documents with n1ql works just fine.
Anyway, here is test code and video.
Test code: https://github.com/moon0326/cb-test (take a look at tests/RequestPlusTest.php)
Video: https://www.youtube.com/watch?v=yhyGULXaoUU&feature=youtu.be