Players accessing inaccessible roof in Breakthrough

Just played a match where the defending team had roof top access on the second location in Mirak Valley, the large tower complex that is only accessible in conquest. I've never seen this accessed before during Breakthrough as all the transport points on the building and the crane are removed in this mode. This made it very difficult to attack, given that they have a position advantage over the attackers spawn area especially when they were using mortars. When inquiring in the chat other players suggested that it was a "drowning glitch" that had allowed them access to this map height.