My Sofirn SP35 makes contact with the battery using a spring in the tail cap and a pad on the driver. It also came with an unprotected 21700 Sofirn branded flat top battery that measures 70.8 mm long.
I'm guessing that your protected 21700 batteries are too long to allow the tail cap to completely screw onto the body of the light which basically creates an "open circuit" between the tail cap and the body of the flashlight and will not allow electrical current to flow to the driver.
When my SP35 is ON, if I unscrew the tail cap even a few degrees, the light will turn OFF.
Without a battery installed, note the rotational orientation of the installed tail cap to the body. Then re-install the battery and determine if you can still screw the tail cap on to the same rotational orientation (while checking that the overall length of the flashlight is the same as before and not any longer).
EDIT: Do not measure the length of your batteries with anything that conducts electricity. I use plastic non-conductive digital calipers.
EDIT #2: Also, make sure the flashlight's body is firmly screwed into the head of the flashlight.
And to test if the problem is from the tail cap not being able to be completely screwed onto the flashlight's body, you could remove the tail cap and use something electrically conductive (like a piece of wire or a paper clip) to "short circuit" and create an electrical path between the negative end of the battery and the shiny non-anodized bare aluminum end (on the cylindrical edge) of the flashlight's body to verify that your SP35 can turn ON using your protected battery without the tail cap.