This bug is not limited to those missions, see there : Gates autodestructing in the second mission of the Jeanne D'Arc campaign
The gates loose HP whenever they open/close themselves after you loaded a save if they are gates that have been given to you from what I’ve seen. That makes the Vilnus Siege especially annoying to play and I’ve been waiting for a patch since the release of the dlc to finally be able to enjoy the campaigns.