While some details are not mention, I don't see that anything went wrong. He lost the signal when the drone was at 60'. Smart RTH kicked in and the drone was set to 300' or so. It started to fly up from 75 to 300'. As it started to fly above that tall building it reconnected. The video is cut but when it returns, the drove is flying back to the home point. What we don't know is if it started to do this as part of RTH or if it rose above the building, the controller reconnected as the building was no longer in the way and it started to RTH on it's own.
But I don't see anything indication that RTH did not work correctly.