The LWS will swap it for a full if you return it right away- light fills happen, and they understand that. It could also be a faulty valve, leaking continuously, so your bottle that appears to be down by just 1/8 may dissipate to nothing. It's your call, but it may also be worth painting the valve with some soapy water, looking for bubbles, then laying the bottle down, valve facing up, and dripping some soapy water in there too.

For me the LWS hours and location are just far enough out of synch with my work schedule and commute that I bring a regulator with me and check the pressure in the new cylinder right there. It's 60 seconds' worth of bother that may prevent a return trip.