CEOs at leading Bitcoin mining firms remain "upbeat" ahead of this week’s halving event despite failing to outperform bitcoin year-to-date, according to research and brokerage firm Bernstein.

The underperformance is due to strong U.S. spot Bitcoin exchange-traded fund flows “sucking away” retail liquidity from miner stocks and concern regarding the halving’s impact on miners’ revenue, Gautam Chhugani and Mahika Sapra wrote in a note to clients on Monday.

According to Bernstein’s recent interviews, Marathon CEO Fred Thiel said the market has seen mining stocks as mere bitcoin proxies so far and that after the ETFs launched, a popular trade has been long spot Bitcoin ETFs and short the miners, explaining the underperformance.

CleanSpark CEO Zack Bradford told Bernstein that Bitcoin …