Ok, I probably misunderstood the description in your first post. My guess is that it’s possible you have two remote zones which are pointing to the same zone on the remote controller. So if you set one of them to run, the other is not running, then the special station auto-refresh will cause the second one to turn off the remote zone after about 2 minutes. You should check through your remote zone settings and see if there might be a duplicate.
The 18 hours run time is normal: it’s the maximum run time allowed on any zone. The actual run time will be decided by the master controller — when the master controller sends a command to turn off that remote zone, it will be shut off. The reason to have a 18 hr maximum is such that in case master controller loses connection, it won’t leave the remote zone on indefinitely.