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Kraft Heinz (KHC - Get Report) shares drifted lower Tuesday, extending a year-to-date decline of around 30%, after billionaire investor Warren Buffett said he 'overpaid' for the packaged food group when he purchased it along with 3G Capital in 2015, but pledged to support incoming CEO Miguel Patricio.

Buffett, who stepped down from the Kraft Heinz board last year, told CNBC that even has he acknowledges the difficulties stemming from his 26.7% stake in the group, and the $3 billion goodwill writedown he was forced to take in February, his relationship with Brazil-based 3G co-founder Jorge Paulo Lemann remains solid and reports of tension between the pair are incorrect.

"I made a mistake in the Kraft purchase in terms of paying too much," Buffett told CNBC, echoing remarks he has made in the past, before adding that both he and Berkshire Hathaway (BRK.A - Get Report) vice chairman Greg Abe were "pleased with the selection of Miguel", who takes over as Kraft Heinz CEO on July 1.

Kraft Heinz shares were marked 0.3% lower Tuesday to change hands  $30.44, a move that would extend its year-to-date decline to around 30% and value the packaged food group at just over $37 billion.

Kraft Heinz said last month it would restate its financial results for 2016 and 2017 following an investigation into some of its accounting practices, but noted the changes wouldn't be "material" and would likely amount to less than $210 million.

Buffett told Berkshire Hathaway's annual shareholder meeting that he wasn't able to book anything from the Kraft Heinz stake in Berkshire's first quarter earnings report owing to the delay in Kraft filing its 10-k with the SEC.

"It's pretty unusual. It means we put in zero for earnings even though we received $130 million in dividends but we don't count that in earnings because it's an equity type of investment, " Buffett said.

Kraft Heinz warned earlier this year that it expects to "take a step backwards" in 2019 after writing down the value of some of its key brands, including Kraft and Oscar Mayer, by as much as $15.4 billion. 

Kraft Heinz has seen a steady decline in market share for its key brands, thanks in part to shifting consumer habits towards healthier options and competition from private label alternatives. It brought forward two years of investment plans into 2018, spending around $300 million in order to drive growth and develop its product pipeline, but fourth quarter net sales still fell shy of analysts forecasts and rose only 0.7% from last year to $6.9 billion.

Kraft also said it had received a subpoena in October "associated with an investigation into" its accounting policies, procedures and internal controls related to its "procurement function."

This includes "agreements, side agreements, and changes or modifications to its agreements with its vendors," the company said. "Following this initial SEC document request, the Company together with external counsel launched an investigation into the procurement area."