Is this a bad cell?

Hi all...I'm relatively new to the high output flashlight scene. With that said, I ordered two keeppower 26650s from Amazon last week and got them in a couple days ago. I ordered them to use in my r50 pro light. I charged both completely on the nitecore d2 charger which took them both to 4.2v. When I put one in the light, turbo mode doesn't work. The other one seems to work fine however. With the bad one, even on high the red low voltage warning on the flashlight comes on. When I took the battery out, it measures 4.18 volts. Is there anything else I can do to test the cell? Also, is this a common problem? Is it likely that the cell is bad or that is something on my end? Thanks a lot.

KeepPower have a pretty good reputation. They are a well-known brand and make quality cells.