An investigation into how a whistleblower's hyponatraemia concerns were handled by officials has found the wrong conclusions were reached.
The concerns were on the adequacy of searches in 2004 on Western Health Board premises for documents relating to the hyponatraemia inquiry.
The inquiry chairman, who ordered the investigation, said there was a failure at leadership level.
Hyponatraemia is a disorder that occurs during a sodium shortage in the blood.
The 14-year hyponatraemia inquiry, chaired by Mr John O'Hara QC, examined the treatment of five children who died in Northern Ireland hospitals between 1995 and 2003.
It concluded that four of the deaths were avoidable.
The whistleblower, who works for the Western Health Trust, raised their concerns in 2018 over the searches of premises belonging to what was then the Western Health and Social Services Board (WHSSB).
An internal inquiry into the handling of those concerns was carried out by the Health and Social Care Board (HSCB) and published last year.
However, a further investigation was ordered by Sir John.
He said he had a number of concerns over the initial investigation including a potential perception of conflict of interest as it had been appointed by the Health and Social Care Board chief executive.
Concerns were also raised that the interviews with witnesses including the whistleblower were "not all conducted appropriately nor were they all properly investigative".
In his report, Sir John, who commended the whistleblower for their actions, said "not only was the whistleblower failed but so too was the service".
It states that the board's investigation failed to state the following:That two of the children who died - Lucy Crawford and Raychel Ferguson - were treated in hospitals within the Western Health and Social Services Board (WHSSB) area That their deaths were both reported contemporaneously to the WHSSB That the Hyponatraemia Inquiry was examining allegations of "cover-up" in relation to their deaths
Sir John added the board's executive summary findings were "wrong" given the evidence before its panel.
While the whistleblower's intervention did not impact on the inquiry's final conclusions, the chairman said it did highlight there were gaps in the WHSSB search for documents in 2004.
He said it also highlighted that the HSCB failed to bring relevant information to the attention of the independent inquiry in 2013.
However, Sir John said the board's failures to identify problems did not constitute a "deliberate attempt to mislead".
The HSCB said it acknowledged Sir John's report contained a "number of issues in relation to how the whistleblowing investigation was carried out".
It added that while the HSCB accepted there was "scope for learning" it also stressed that panel members "carried out a very complex investigation within a constrained timeline with honesty, integrity and dedication".
"The panel members were acutely aware of the seriousness of the whistleblower's concerns, given the tragic circumstances which led to the hyponatraemia inquiry, and strived to consider the issues raised in a fair and impartial manner.
"The HSCB is firmly committed to working with the wider healthcare system to ensure that any learning is fully taken on board."