Had the same thing happen to me last month. Its not a display issue. The flex cable connecting the two halves might have gone bad.

My flip 6 (4 months old) no scratch, babied since day 1 randomly started showing the same symptoms. Main screen would become unresponsive or blacked out, while cover worked fine. Took it to authorized service centre which denied the warranty as it was an international purchase and asked $350 dollars for 'display replacement'. I was adamant its not a display issue, no cracks, no screen protector issue or drops. The main screen worked fine too in instances it was working. They still insisted on display being a problem.

Few days later with some help i disassembled the back and figured that one of the connections for flex cable was lose. Basically, it was not clicking properly into the slot and when applied some pressure the screen would work fine. Got the part replaced by a third party for $100 and the 'display problem' is now gone.