The actual function is considering active auctions as expired (if endTimestamp is in the future)

 function isAuctionExpired(uint256 _auctionId) external view onlyExistingAuction(_auctionId) returns (bool) { return _englishAuctionsStorage().auctions[_auctionId].endTimestamp >= block.timestamp; } 

The correct conditional should be to check if endTimestamp is in the past, and then return true for expired.

I opened a PR with the fix here