IMO part of the problem is that even many unprotected 18650 batteries, particularly the highest capacity ones, are over 18mm in diameter for the base wrapped battery. The makers are cheating from the start in order to maximize internal volume to get in as much active material as possible. I personally have yet to measure an 18650 that is only 18.0mm in diameter. Not sure they exist in current batteries.
Zebralight is not alone in this battery fit problem as I have a Jetbeam that takes a three 18650 battery holder and some 18650 batteries are too large in diameter to allow the battery holder to fit in the battery tube. Length can also be a problem as some lights are designed to only take unprotected 65mm batteries and some protected Lithium ion 18650s may be over 70mm in length.
There are NO STANDARDS for maximum size of either unprotected or protected Lithium Ion batteries that I am aware of and until there are there will likely continue to be intermittent fit problems. If light makers make their lights to take the largest and longest batteries of a given nominal size then there will be rattle and possible length related contact problems with the smallest diameter and shortest ones. The Nitecore TM 15 and TM 26 both only take quite limited lengths of protected 18650 batteries due to the design of the negative contact spring in their battery tubes. Neither unprotected 18650 batteries or the longest protected ones fit the lights and per what I have read the longest protected batteries can damage the spring plate.