Yeah, this is a common point of confusion. A slow, periodic trickle or drip from the drain line is normal during the filtration cycle—that's the wastewater being discharged.
What's not normal is a continuous stream or flow when the system should be in standby mode (tank full). That means the automatic shut-off valve isn't working, likely due to low incoming water pressure or a faulty valve.
First, check that your home's water pressure is within spec. If pressure is too low, the valve can't seal properly.
Second, inspect the drain line installation. The drain saddle must be securely mounted on the horizontal section of your sink's drain pipe, and the 1/4" tubing must be inserted fully into the saddle. If it's on a vertical pipe or not sealed, it can siphon or leak.
If you've got good pressure and a proper drain connection but it still flows non-stop, the shut-off valve inside the RO head may need service.