Details of Nadeem Ahmed Kiani's convictions emerged in August, leading to Lib Dem Subhan Shafiq resigning as Milton Keynes mayor.
Fellow party member Stuart Burke resigned as chairman of Milton Keynes Council's licensing committee.
The decision to keep the investigation secret has been called "disrespectful".
Milton Keynes Council revoked Kiani's licence last year after it was revealed he had been given a private hire licence by the licensing committee in 2011.
A second committee lifted a suspension on his licence in 2012, despite Thames Valley Police having written to the authority giving more details of Kiani's convictions for raping and assaulting prostitutes in London in 1994.
Mr Shafiq, who had vouched for Kiani as a friend in 2011, resigned as mayor in August. He quit as a councillor when the council released a report into the affair in November.
Mr Burke, who had been chairman of the second committee, resigned from that role along with vice-chairman and Labour councillor Gladstone McKenzie.
The completion of the internal investigation at the start of December came to light following repeated inquiries from the BBC.
Lib Dem campaign manager Jane Carr said recommendations had been "locally actioned with the respective individuals fully complying" with them. She said the report was "confidential to those involved and will not be released into the public domain".
Rosemary Smith, founder of the Milton Keynes Taxi Association, said the decision to keep the results private was "appalling".
Former council leader, Conservative Andrew Geary, said: "What a surprise - the Lib Dem stitch-up strikes again."
