My suspicion is that your stopper isn’t sealing well, possibly from wear or just odd positioning. If you feel like ruining what’s left of that Jim beam in the decanter you can run a little experiment. Clear the condensation out of the decanter and mark the current level. Leave it for a week and see if the level dropped to establish a baseline. After marking the change, coat the stopper in Vaseline and leave it for another week. See if that helped or not. If it helped then you’ve got a leak