Flat Bug - Acantharadus quaternarius

Had originally thought this is an Assassin Bug, so was pleasantly surprised to get an ID and is a Flat Bug, seems to be a rare find too. Saw several of them, and then noticed that the specimen on the right (darker colour) is already adult and the specimen on the left (lighter colour) is a sub-adult. Have another picture with 2 adults with 1 sub-adult, and I gather the sub-adult is a female and the adults were likely males, waiting for the female to emerge as adult. Saw them for 3 days at the same tree trunk and then on the 4th day, all of them were gone, probably moved away after the female is adult and have mated.