Does somebody know how I can understand the issue if disambiguation is applied incorrectly?

Given a CSL with such an entry:
<citation et-al-min="4" et-al-use-first="1" disambiguate-add-year-suffix="true" disambiguate-add-names="true" disambiguate-add-givenname="true" givenname-disambiguation-rule="primary-name" collapse="year">

I'm seeing an entry such as 2016b, yet it's the only entry for 2016, the only entry for that author and there's no 2016a.

Any idea on how I can understand the root cause?

Example see: