Thanks for reporting this pKa bug.
Sorry for the week long lack of answer but unfortunately I received your question only today due to some technical issue.
Based on your feedback an improved version will be released asap, probably on the next week.
The pKa value of a "similar" molecule and its experimental pKa value is given on the attached figure.
The carbanion species will be present in circa 7% at pH=7.2 according to the predicted pKa value of the molecule you brought up in this discussion.
Hoping we were able to help you.