It’s the diameter. You and I must not use the same protected cells or this isn’t a clone. I use some unknown laptop pulls in my light. I just tried protected Xtar 2600s and the popular Sanyo red protected from fastechs (2600s too) and neither would even enter the tube.
The product page for the S6 says it will not accept protected cells. So I dunno.
Also post #38 of this thread, says his light does not take them either- while others report them having worked.