I'm not sure if this is a bug. If you all go through the portal single file and wait until the "chime" ends between each exit or entrance you will not have an issue. I have noticed that this bug occurs, if it is a bug, only when two guardians run the the portal at the same time. Try it out and assign roles for exit. My team goes Relic holder first, then one followed by the other all entering the cleansing bubble upon exit put down by the relic holder who went through first. Hope this helps!