A review into the murder of Rhys Jones, who was killed by a man released five days earlier, questioned whether his killer's risk to the public should have been reassessed.
But Mr Jones' family were initially blocked from reading it.
Plans to transfer the management of such reviews to HM Probation have been put on hold.
Mr Jones, 36, who had schizophrenia and had been under the care of mental health services for much of his life, was stabbed to death by Rhys Barnes in November 2015 at the Solas supported housing hostel where they were living in Newport.
Barnes, 28, who had 17 previous convictions for 41 separate offences including grievous bodily harm, assault and carrying offensive weapons, was jailed for life in March 2016.
At the time of the killing he was being supervised by the National Probation Service after his release from prison, having been assessed as a high risk to his family and a medium risk to the public.
He had been readmitted to prison at the beginning of November for breaching supervision conditions, but five days after coming out, he murdered Rhys Jones following a row about a girlfriend.
A review into the killing questioned whether Barnes' level of risk to the general public should have been reassessed, and whether a multi-agency public protection meeting should have been held.
The report said staff should consider referring prisoners who breach their licence conditions to a probation-approved premises following re-release.
The findings of the review were shared with staff across Wales, "to highlight areas of learning so that staff across the organisation can embed the learning into their own practice".
But Mr Jones' family had to push to see it after they claim they were initially told it "would not benefit" them to know the findings.
David Hanson, Labour MP for Delyn and a member of the justice select committee in Westminster, said it was important families and the public know what went wrong in cases such as these.
"That means that the report has to be published for the victim, but also for those like myself who take an interest to make sure the system is working well," he said.
"Without that we think something may well be hidden and something may not be dealt with that could be dealt with."
Labour's Newport East AM John Griffiths, who had been supporting Mr Jones' family, echoed his call.
"I would pay tribute to Rhys' family because they've shown dogged determination to try and ensure that the whole of the circumstances are understood, and that lessons are learnt for the future, crucially, to try and minimise the chances of further tragic loss of life in these sorts of circumstances," he said.
"The more openness the better, for public confidence; if people feel things are being hidden that arouses suspicions and concern."
Tania Bassett, of the National Association of Probation Officers, called on the government to honour its pledge to transfer management of serious further offence reviews from the probation service to Her Majesty's Inspectorate of Probation.
She said: "We welcome that [the transfer] because it would apply a much more consistent approach across the board: everyone would have the same treatment, it would be an external independent body looking at that investigation rather than those who may have their own commercial interests, and also it would mean greater transparency for the public, which we think is the right way forward."
The National Probation Service said: "Serious further offences such as this [Mr Jones's murder] are rare but each one is taken extremely seriously and investigated fully.
"Public protection is our priority. A full review was undertaken and all identified learning was taken forward."
Nick Taylor, of Solas Cymru, said an internal hostel review of Mr Jones' killing found there was "absolutely no way that this incident could have been predicted".
Inquiries into serious crimes committed by recently-released prisoners should be published, politicians have said.