Experts have questioned the reason for a share purchase by Wrightbus's parent company at the time the group was in financial trouble.
The Wrightbus group collapsed into administration in September 2019.
In April 2019 the group parent company, Cornerstone, spent £1m buying back the shares of a Wright family member.
Forensic accountant Gabriel Greene told BBC Spotlight: "I'm struggling to find a commercial reality to that transaction."
The deal concerned a 5% shareholding in Cornerstone held by Lorraine Rock.
Dr Rock is a sister of Jeff Wright, who is the controlling shareholder of Cornerstone.
'Acted lawfully at all times'
Mr Greene said the deal would have valued the group at potentially up to £80m.
"This is in the context of a very, very heavily loss making, and potentially, at this stage, insolvent group," he told Spotlight.
"So I'm struggling to comprehend how that valuation was got to, at that point, and why that payment was made."
Spotlight wrote to Dr Rock and other members of the Wright family with an interest in Cornerstone to ask them to explain why the transaction took place.
They did not respond to that question, but did say they had acted appropriately and lawfully at all times.
A commercial lawyer, Mark Orr QC, told the programme the circumstances in which a company buys its own shares are either where there has been a company dispute or where there is surplus capital which it wants to pay out to shareholders.
"There doesn't appear to be evidence of either in the documentation which I've seen to date. I can find no reason for the purchase of own shares at the time when it was effected," he said.
'Paradoxical kind of situation'
Earlier in 2019 Cornerstone also paid out £1.35m in a donation to Green Pastures church, which is led by Jeff Wright.
Within months the Wrightbus group approached Invest NI to ask for public funds to keep the business afloat.
Invest NI agreed to release an initial tranche of £2.5m in rescue aid.
A corporate governance expert told Spotlight there was "a curious equivalence" between the money which was paid out and the £2.5m which was then requested.
Clive Grace, a former director-general of the Welsh Audit Commission, said: "The fact that public money was being injected shortly after a similar sum was being extracted from the parent Cornerstone is a very ironic, paradoxical kind of situation."
According to the Wright family, any donations paid by Cornerstone were made from available profits with the knowledge of Invest NI and the Bank of Ireland, its lender.
Invest NI said the rationale behind granting £2.5m in rescue aid was that it "offered the best prospect of finding a buyer for the business".
The organisation declined to comment on the activities of the company directors in Wrightbus and the wider group - given the administrators Deloitte are currently conducting a review into their conduct.
Mr Grace suggested the NI Audit Office, the public spending watchdog, should look at the issue of the rescue aid.
"They would be able to see all the documentation, they would be able to interview people from Invest NI, they would be able to thoroughly see whether more things should have been taken into account, whether there are lessons to learn," he said.
"And in particular are there implications for the future rules and decision making when it comes to this kind of development assistance?"
You can watch Spotlight on the BBC iPlayer.