Maybe we should just all cut and paste from the Cash Acme literature and say "The SharkBite® fittings have an integral Tube Support Liner that is effective in supporting PEX tubing to ensure the o-ring seals correctly on the outside of the PEX tubing." So WHAT it does is ensure that the o-ring seals and HOW it does it is by supporting the PEX tubing. WHY it does it is because the o-ring can compress the pipe and cause a leak.
Interestingly, my nephew discovered that the liner had pushed into the fitting while correcting a leak at a pipe nipple, which he discovered during the pressure test. He pulled the tube support liner, corrected the leak at the nipple without addressing the issue with the liner, and re-ran the 24 hour pressure test with positive results. He still plans on changing out the fitting, figuring it's not worth trying to repair it just to save $8. After all, it's hard to say at this point if the issue caused by the installer, a manufacturing defect, or damage to the fitting.
Thanks again to all who took the time to reply.